diff --git a/lib/functions_framework/legacy_event_converter.rb b/lib/functions_framework/legacy_event_converter.rb
index 84c15160..5d0af0a0 100644
--- a/lib/functions_framework/legacy_event_converter.rb
+++ b/lib/functions_framework/legacy_event_converter.rb
@@ -135,18 +135,22 @@ def convert_source service, resource, domain
 
       match = CE_SERVICE_TO_RESOURCE_RE[service].match resource
       return [nil, nil] unless match
+      resource_fragment = match[1]
+      subject = match[2]
 
       if service == "firebasedatabase.googleapis.com"
-        return [nil, nil] if domain.nil?
-        location = "us-central1"
-        if domain != "firebaseio.com"
-          location_match = domain.match(/^([\w-]+)\.firebasedatabase\.app$/)
-          return [nil, nil] unless location_match
-          location = location_match[1]
-        end
-        ["//#{service}/projects/_/locations/#{location}/#{match[1]}", match[2]]
+        location =
+          case domain
+          when "firebaseio.com"
+            "us-central1"
+          when /^([\w-]+)\./
+            Regexp.last_match[1]
+          else
+            return [nil, nil]
+          end
+        ["//#{service}/projects/_/locations/#{location}/#{resource_fragment}", subject]
       else
-        ["//#{service}/#{match[1]}", match[2]]
+        ["//#{service}/#{resource_fragment}", subject]
       end
     end
 
diff --git a/test/test_legacy_event_converter.rb b/test/test_legacy_event_converter.rb
index 4c0fd0f1..cd2a9baa 100644
--- a/test/test_legacy_event_converter.rb
+++ b/test/test_legacy_event_converter.rb
@@ -13,16 +13,27 @@
 # limitations under the License.
 
 require "helper"
+require "json"
+require "stringio"
 
 describe FunctionsFramework::LegacyEventConverter do
   let(:data_dir) { File.join __dir__, "legacy_events_data" }
 
-  def load_legacy_event filename, url_path: nil, encoding: "utf-8"
-    path = File.join data_dir, filename
-    File.open path, encoding: encoding do |io|
+  def load_legacy_event filename_or_json, url_path: nil, encoding: "utf-8"
+    converter = FunctionsFramework::LegacyEventConverter.new
+    case filename_or_json
+    when String
+      path = File.join data_dir, filename_or_json
+      File.open path, encoding: encoding do |io|
+        env = { "rack.input" => io, "CONTENT_TYPE" => "application/json", "PATH_INFO" => url_path }
+        converter.decode_rack_env env
+      end
+    when Hash
+      io = StringIO.new JSON.dump filename_or_json
       env = { "rack.input" => io, "CONTENT_TYPE" => "application/json", "PATH_INFO" => url_path }
-      converter = FunctionsFramework::LegacyEventConverter.new
       converter.decode_rack_env env
+    else
+      raise ArgumentError, filename_or_json.class.name
     end
   end
 
@@ -218,4 +229,26 @@ def load_legacy_event filename, url_path: nil, encoding: "utf-8"
     assert_equal "refs/gcf-test/abc", event.subject
     assert_equal "2020-05-21T11:56:12+00:00", event.time.rfc3339
   end
+
+  it "declines to convert firebasedatabase without a domain" do
+    json = {
+      "eventType" => "providers/google.firebase.database/eventTypes/ref.write",
+      "params" => {
+        "child" => "xyz"
+      },
+      "auth" => {
+        "admin" => true
+      },
+      "data" => {
+        "data" => nil,
+        "delta" => {
+          "grandchild" => "other"
+        }
+      },
+      "resource" => "projects/_/instances/my-project-id/refs/gcf-test/xyz",
+      "timestamp" => "2020-05-21T11:15:34.178Z",
+      "eventId" => "/SnHth9OSlzK1Puj85kk4tDbF90="
+    }
+    assert_nil load_legacy_event json
+  end
 end