diff --git a/src/Guesser/StubAsGuesser/StubAsBackedEnumGuesser.php b/src/Guesser/StubAsGuesser/StubAsBackedEnumGuesser.php
index 71d3d67..5f45906 100644
--- a/src/Guesser/StubAsGuesser/StubAsBackedEnumGuesser.php
+++ b/src/Guesser/StubAsGuesser/StubAsBackedEnumGuesser.php
@@ -6,6 +6,7 @@
 use Ingenerator\StubObjects\Attribute\StubAs;
 use Ingenerator\StubObjects\Attribute\StubAs\StubAsBackedEnum;
 use Ingenerator\StubObjects\Guesser\StubAsGuesser;
+use Ingenerator\StubObjects\ReflectionUtils;
 use Ingenerator\StubObjects\StubbingContext;
 use ReflectionProperty;
 
@@ -13,16 +14,12 @@ class StubAsBackedEnumGuesser implements StubAsGuesser
 {
     public function guessCaster(ReflectionProperty $property, StubbingContext $context): false|StubAs
     {
-        if ( ! $property->getType()) {
+        if (ReflectionUtils::isBuiltinType($property)) {
             return FALSE;
         }
 
-        if ($property->getType()->isBuiltin()) {
-            return FALSE;
-        }
-
-        $type = $property->getType()->getName();
-        if (is_a($type, BackedEnum::class, TRUE)) {
+        $type = ReflectionUtils::getTypeNameIfAvailable($property);
+        if ($type && is_a($type, BackedEnum::class, TRUE)) {
             return new StubAsBackedEnum($type);
         }
 
diff --git a/src/Guesser/StubAsGuesser/StubAsCollectionGuesser.php b/src/Guesser/StubAsGuesser/StubAsCollectionGuesser.php
index b4a768b..efde234 100644
--- a/src/Guesser/StubAsGuesser/StubAsCollectionGuesser.php
+++ b/src/Guesser/StubAsGuesser/StubAsCollectionGuesser.php
@@ -6,6 +6,7 @@
 use Ingenerator\StubObjects\Attribute\StubAs;
 use Ingenerator\StubObjects\Attribute\StubAs\StubAsCollection;
 use Ingenerator\StubObjects\Guesser\StubAsGuesser;
+use Ingenerator\StubObjects\ReflectionUtils;
 use Ingenerator\StubObjects\StubbingContext;
 use ReflectionProperty;
 
@@ -13,8 +14,7 @@ class StubAsCollectionGuesser implements StubAsGuesser
 {
     public function guessCaster(ReflectionProperty $property, StubbingContext $context): false|StubAs
     {
-        // @todo: test with untyped props
-        if ($property->getType()?->getName() === Collection::class) {
+        if (ReflectionUtils::getTypeNameIfAvailable($property) === Collection::class) {
             $collected_class = $this->parseItemClass($property);
 
             return new StubAsCollection($property->getType()->getName(), $collected_class);
diff --git a/src/Guesser/StubAsGuesser/StubAsDateTimeGuesser.php b/src/Guesser/StubAsGuesser/StubAsDateTimeGuesser.php
index 04edcbc..8400546 100644
--- a/src/Guesser/StubAsGuesser/StubAsDateTimeGuesser.php
+++ b/src/Guesser/StubAsGuesser/StubAsDateTimeGuesser.php
@@ -6,6 +6,7 @@
 use Ingenerator\StubObjects\Attribute\StubAs;
 use Ingenerator\StubObjects\Attribute\StubAs\StubAsDateTime;
 use Ingenerator\StubObjects\Guesser\StubAsGuesser;
+use Ingenerator\StubObjects\ReflectionUtils;
 use Ingenerator\StubObjects\StubbingContext;
 use ReflectionProperty;
 
@@ -13,8 +14,7 @@ class StubAsDateTimeGuesser implements StubAsGuesser
 {
     public function guessCaster(ReflectionProperty $property, StubbingContext $context): false|StubAs
     {
-        // @todo test with untyped props
-        if ($property->getType()?->getName() === DateTimeImmutable::class) {
+        if (ReflectionUtils::getTypeNameIfAvailable($property) === DateTimeImmutable::class) {
             return new StubAsDateTime();
         }
 
diff --git a/src/Guesser/StubAsGuesser/StubAsStubObjectGuesser.php b/src/Guesser/StubAsGuesser/StubAsStubObjectGuesser.php
index ef58a8c..3ae8572 100644
--- a/src/Guesser/StubAsGuesser/StubAsStubObjectGuesser.php
+++ b/src/Guesser/StubAsGuesser/StubAsStubObjectGuesser.php
@@ -5,6 +5,7 @@
 use Ingenerator\StubObjects\Attribute\StubAs;
 use Ingenerator\StubObjects\Attribute\StubAs\StubAsStubObject;
 use Ingenerator\StubObjects\Guesser\StubAsGuesser;
+use Ingenerator\StubObjects\ReflectionUtils;
 use Ingenerator\StubObjects\StubbingContext;
 use ReflectionProperty;
 
@@ -12,12 +13,11 @@ class StubAsStubObjectGuesser implements StubAsGuesser
 {
     public function guessCaster(ReflectionProperty $property, StubbingContext $context): false|StubAs
     {
-        // @todo: test with untyped props
-        if ($property->getType()?->isBuiltin()) {
+        if (ReflectionUtils::isBuiltinType($property)) {
             return FALSE;
         }
 
-        $class = $property->getType()?->getName();
+        $class = ReflectionUtils::getTypeNameIfAvailable($property);
         if ($class && $context->isStubbable($class)) {
             return new StubAsStubObject($class);
         }
diff --git a/src/Guesser/StubDefaultGuesser/StubDefualtDateTimeGuesser.php b/src/Guesser/StubDefaultGuesser/StubDefaultDateTimeGuesser.php
similarity index 79%
rename from src/Guesser/StubDefaultGuesser/StubDefualtDateTimeGuesser.php
rename to src/Guesser/StubDefaultGuesser/StubDefaultDateTimeGuesser.php
index 684a0c3..44fedaf 100644
--- a/src/Guesser/StubDefaultGuesser/StubDefualtDateTimeGuesser.php
+++ b/src/Guesser/StubDefaultGuesser/StubDefaultDateTimeGuesser.php
@@ -6,14 +6,15 @@
 use Ingenerator\StubObjects\Attribute\StubDefault;
 use Ingenerator\StubObjects\Attribute\StubDefault\StubDefaultValue;
 use Ingenerator\StubObjects\Guesser\StubDefaultGuesser;
+use Ingenerator\StubObjects\ReflectionUtils;
 use Ingenerator\StubObjects\StubbingContext;
 use ReflectionProperty;
 
-class StubDefualtDateTimeGuesser implements StubDefaultGuesser
+class StubDefaultDateTimeGuesser implements StubDefaultGuesser
 {
     public function guessProvider(ReflectionProperty $property, StubbingContext $context): false|StubDefault
     {
-        if ($property->getType()->getName() === DateTimeImmutable::class) {
+        if (ReflectionUtils::getTypeNameIfAvailable($property) === DateTimeImmutable::class) {
             // Note that we're guessing this in string form, converting back to a DateTime happens at the
             // point of hydrating values because we anyway need to do it there for values that came in overrides
             return new StubDefaultValue('now');
diff --git a/src/Guesser/StubDefaultGuesser/StubDefaultStringGuesser.php b/src/Guesser/StubDefaultGuesser/StubDefaultStringGuesser.php
index d7c8a0d..3608b44 100644
--- a/src/Guesser/StubDefaultGuesser/StubDefaultStringGuesser.php
+++ b/src/Guesser/StubDefaultGuesser/StubDefaultStringGuesser.php
@@ -5,6 +5,7 @@
 use Ingenerator\StubObjects\Attribute\StubDefault;
 use Ingenerator\StubObjects\Attribute\StubDefault\StubDefaultRandomString;
 use Ingenerator\StubObjects\Guesser\StubDefaultGuesser;
+use Ingenerator\StubObjects\ReflectionUtils;
 use Ingenerator\StubObjects\StubbingContext;
 use Random\Randomizer;
 use ReflectionProperty;
@@ -19,7 +20,7 @@ public function __construct(
 
     public function guessProvider(ReflectionProperty $property, StubbingContext $context): false|StubDefault
     {
-        if ($property->getType()->getName() === 'string') {
+        if (ReflectionUtils::getTypeNameIfAvailable($property) === 'string') {
             // @todo: guess email addresses for props with `email` in the name
             // @todo: guess URLs for props with `url` in the name
             return new StubDefaultRandomString(
diff --git a/src/Guesser/StubDefaultGuesser/StubDefaultStubbableObjectGuesser.php b/src/Guesser/StubDefaultGuesser/StubDefaultStubbableObjectGuesser.php
index 6fe4330..fe30c95 100644
--- a/src/Guesser/StubDefaultGuesser/StubDefaultStubbableObjectGuesser.php
+++ b/src/Guesser/StubDefaultGuesser/StubDefaultStubbableObjectGuesser.php
@@ -6,6 +6,7 @@
 use Ingenerator\StubObjects\Attribute\StubDefault;
 use Ingenerator\StubObjects\Attribute\StubDefault\StubDefaultValue;
 use Ingenerator\StubObjects\Guesser\StubDefaultGuesser;
+use Ingenerator\StubObjects\ReflectionUtils;
 use Ingenerator\StubObjects\StubbingContext;
 use ReflectionProperty;
 
@@ -13,7 +14,7 @@ class StubDefaultStubbableObjectGuesser implements StubDefaultGuesser
 {
     public function guessProvider(ReflectionProperty $property, StubbingContext $context): false|StubDefault
     {
-        if ($property->getType()->isBuiltin()) {
+        if (ReflectionUtils::isBuiltinType($property)) {
             return FALSE;
         }
 
@@ -29,18 +30,19 @@ public function guessProvider(ReflectionProperty $property, StubbingContext $con
 
     private function isStubbable(StubbingContext $context, ReflectionProperty $property): bool
     {
-        $type = $property->getType();
-        if ($type->isBuiltin()) {
+        $name = ReflectionUtils::getTypeNameIfAvailable($property);
+
+        if ($name === FALSE) {
             return FALSE;
         }
 
-        if ($type->getName() === Collection::class) {
+        if ($name === Collection::class) {
             // Treat collections as a special case, we'll recursively attempt to cast arrays to collections
             return TRUE;
         }
 
         // Otherwise it depends if they've configured recursion
-        return $context->isStubbable($property->getType()->getName());
+        return $context->isStubbable($name);
     }
 
 }
diff --git a/src/ReflectionUtils.php b/src/ReflectionUtils.php
new file mode 100644
index 0000000..540b46a
--- /dev/null
+++ b/src/ReflectionUtils.php
@@ -0,0 +1,27 @@
+<?php
+
+namespace Ingenerator\StubObjects;
+
+use ReflectionNamedType;
+use ReflectionProperty;
+
+class ReflectionUtils
+{
+
+    public static function isBuiltinType(ReflectionProperty $property): bool
+    {
+        $type = $property->getType();
+
+        return ($type instanceof ReflectionNamedType) && $type->isBuiltin();
+    }
+
+    public static function getTypeNameIfAvailable(ReflectionProperty $property): false|string
+    {
+        $type = $property->getType();
+        if ($type instanceof \ReflectionNamedType) {
+            return $type->getName();
+        }
+
+        return FALSE;
+    }
+}
diff --git a/src/StandardConfig.php b/src/StandardConfig.php
index 76b5f94..39859fe 100644
--- a/src/StandardConfig.php
+++ b/src/StandardConfig.php
@@ -8,10 +8,10 @@
 use Ingenerator\StubObjects\Guesser\StubAsGuesser\StubAsDateTimeGuesser;
 use Ingenerator\StubObjects\Guesser\StubAsGuesser\StubAsStubObjectGuesser;
 use Ingenerator\StubObjects\Guesser\StubDefaultGuesser;
+use Ingenerator\StubObjects\Guesser\StubDefaultGuesser\StubDefaultDateTimeGuesser;
 use Ingenerator\StubObjects\Guesser\StubDefaultGuesser\StubDefaultNullGuesser;
 use Ingenerator\StubObjects\Guesser\StubDefaultGuesser\StubDefaultStringGuesser;
 use Ingenerator\StubObjects\Guesser\StubDefaultGuesser\StubDefaultStubbableObjectGuesser;
-use Ingenerator\StubObjects\Guesser\StubDefaultGuesser\StubDefualtDateTimeGuesser;
 
 class StandardConfig
 {
@@ -36,7 +36,7 @@ public static function loadDefaultValueGuessers(): array
     {
         return [
             new StubDefaultNullGuesser(),
-            new StubDefualtDateTimeGuesser(),
+            new StubDefaultDateTimeGuesser(),
             new StubDefaultStringGuesser(),
             new StubDefaultStubbableObjectGuesser(),
         ];
diff --git a/test/integration/HandlesUnsupportedTypesTest.php b/test/integration/HandlesUnsupportedTypesTest.php
new file mode 100644
index 0000000..d308478
--- /dev/null
+++ b/test/integration/HandlesUnsupportedTypesTest.php
@@ -0,0 +1,46 @@
+<?php
+
+namespace test\integration;
+
+use Ingenerator\StubObjects\Attribute\StubDefault\StubDefaultValue;
+use Ingenerator\StubObjects\StubObjects;
+use PHPUnit\Framework\Attributes\TestWith;
+use PHPUnit\Framework\TestCase;
+
+class HandlesUnsupportedTypesTest extends TestCase
+{
+
+    #[TestWith([[], ['something' => 'default']])]
+    #[TestWith([['something' => 'whatever'], ['something' => 'whatever']])]
+    public function test_it_can_stub_objects_with_union_types(array $data, array $expect)
+    {
+        $class = new class {
+            #[StubDefaultValue('default')]
+            public bool|string $something;
+        };
+
+        $result = $this->newSubject()->stub($class::class, $data);
+
+        $this->assertSame($expect, (array) $result);
+    }
+
+    #[TestWith([[], ['something' => null]])]
+    #[TestWith([['something' => 'whatever'], ['something' => 'whatever']])]
+    public function test_it_can_stub_objects_with_untyped_properties(array $data, array $expect)
+    {
+        $class = new class {
+            // Note: this will be auto-detected as able to be stubbed with `null`, which is technically correct.
+            public $something;
+        };
+
+        $result = $this->newSubject()->stub($class::class, $data);
+
+        $this->assertSame($expect, (array) $result);
+    }
+
+
+    private function newSubject(array $stubbable_class_patterns = ['*']): StubObjects
+    {
+        return new StubObjects(...get_defined_vars());
+    }
+}
diff --git a/test/unit/Guesser/StubDefault/StubDefaultDateTimeGuesserTest.php b/test/unit/Guesser/StubDefault/StubDefaultDateTimeGuesserTest.php
index 251e426..e05c96a 100644
--- a/test/unit/Guesser/StubDefault/StubDefaultDateTimeGuesserTest.php
+++ b/test/unit/Guesser/StubDefault/StubDefaultDateTimeGuesserTest.php
@@ -5,7 +5,7 @@
 
 use DateTimeImmutable;
 use Ingenerator\StubObjects\Attribute\StubDefault\StubDefaultValue;
-use Ingenerator\StubObjects\Guesser\StubDefaultGuesser\StubDefualtDateTimeGuesser;
+use Ingenerator\StubObjects\Guesser\StubDefaultGuesser\StubDefaultDateTimeGuesser;
 
 class StubDefaultDateTimeGuesserTest extends BaseStubDefaultGuesserTestCase
 {
@@ -22,7 +22,7 @@ public function test_it_guesses_current_time()
                 'string' => FALSE,
             ],
             $class::class,
-            new StubDefualtDateTimeGuesser,
+            new StubDefaultDateTimeGuesser,
         );
     }
 }