diff --git a/src/dioptra/task_engine/error_message.py b/src/dioptra/task_engine/error_message.py
index 018197b38..e3723c753 100644
--- a/src/dioptra/task_engine/error_message.py
+++ b/src/dioptra/task_engine/error_message.py
@@ -259,7 +259,7 @@ def _is_valid_for_sub_schema(
         resolver=jsonschema.validators.RefResolver.from_schema(full_schema),
     )
 
-    return cast(bool, validator.is_valid(sub_instance))
+    return validator.is_valid(sub_instance)
 
 
 def _one_of_too_many_alternatives_satisfied_message_lines(
@@ -280,11 +280,15 @@ def _one_of_too_many_alternatives_satisfied_message_lines(
         in another, with indented lines.
     """
 
-    alt_names = _get_one_of_alternative_names(error.validator_value, schema)
+    # In this specific case, validator_value contains the oneOf alternative
+    # schemas, but the typing is such that mypy can't tell that's the case.
+    alt_schemas = cast(Iterable[Any], error.validator_value)
+
+    alt_names = _get_one_of_alternative_names(alt_schemas, schema)
     error_desc = "Must be exactly one of: {}".format(", ".join(alt_names))
 
     satisfied_alt_names = []
-    for alt_name, alt_schema in zip(alt_names, error.validator_value):
+    for alt_name, alt_schema in zip(alt_names, alt_schemas):
         # Perform a little "mini" validation to determine which alternatives
         # were satisfied, and describe them in the error message.
         if _is_valid_for_sub_schema(schema, alt_schema, error.instance):
@@ -323,9 +327,13 @@ def _one_of_no_alternatives_satisfied_message_lines(
 
     message_lines = []
 
+    # In this specific case, validator_value contains the oneOf alternative
+    # schemas, but the typing is such that mypy can't tell that's the case.
+    alt_schemas = cast(Iterable[Any], error.validator_value)
+
     # First error message line describes the error in basic terms.
 
-    alt_names = _get_one_of_alternative_names(error.validator_value, schema)
+    alt_names = _get_one_of_alternative_names(alt_schemas, schema)
     basic_desc = (
         "Must be exactly one of: {}; all alternatives failed validation."
     ).format(", ".join(alt_names))
diff --git a/tox.ini b/tox.ini
index 01181b104..6df5e06f7 100644
--- a/tox.ini
+++ b/tox.ini
@@ -180,6 +180,7 @@ deps =
     mypy_extensions
     types-cryptography
     types-freezegun
+    types-jsonschema
     types-mypy-extensions
     types-python-dateutil
     types-PyYAML