diff --git a/src/django_mysql/models/fields/dynamic.py b/src/django_mysql/models/fields/dynamic.py
index 287b7ae9..2bd7f099 100644
--- a/src/django_mysql/models/fields/dynamic.py
+++ b/src/django_mysql/models/fields/dynamic.py
@@ -179,13 +179,18 @@ def _check_spec_recursively(
                 subpath = f"{path}.{key}"
                 errors.extend(self._check_spec_recursively(value, subpath))
             elif value not in KeyTransform.SPEC_MAP:
+                valid_names = ", ".join(
+                    sorted(x.__name__ for x in KeyTransform.SPEC_MAP.keys())
+                )
                 errors.append(
                     checks.Error(
                         "The value for '{}' in 'spec{}' is not an allowed type".format(
                             key, path
                         ),
-                        hint="'spec' values must be one of the following "
-                        "types: {}".format(KeyTransform.SPEC_MAP_NAMES),
+                        hint=(
+                            "'spec' values must be one of the following types: "
+                            + valid_names
+                        ),
                         obj=self,
                         id="django_mysql.E011",
                     )
@@ -306,10 +311,8 @@ class KeyTransform(Transform):
         dict: "BINARY",
     }
 
-    SPEC_MAP_NAMES = ", ".join(sorted(x.__name__ for x in SPEC_MAP.keys()))
-
-    TYPE_MAP: dict[str, type[Field] | Field] = {
-        "BINARY": DynamicField,
+    TYPE_MAP: dict[str, Field[Any, Any]] = {
+        # Excludes BINARY -> DynamicField as that’s requires spec
         "CHAR": TextField(),
         "DATE": DateField(),
         "DATETIME": DateTimeField(),
@@ -322,23 +325,22 @@ def __init__(
         self,
         key_name: str,
         data_type: str,
-        *args: Any,
+        *expressions: Any,
         subspec: SpecDict | None = None,
-        **kwargs: Any,
     ) -> None:
-        super().__init__(*args, **kwargs)
-        self.key_name = key_name
-        self.data_type = data_type
-
-        try:
-            output_field = self.TYPE_MAP[data_type]
-        except KeyError:  # pragma: no cover
-            raise ValueError(f"Invalid data_type '{data_type}'")
-
+        output_field: Field[Any, Any]
         if data_type == "BINARY":
-            self.output_field = output_field(spec=subspec)
+            output_field = DynamicField(spec=subspec)
         else:
-            self.output_field = output_field
+            try:
+                output_field = self.TYPE_MAP[data_type]
+            except KeyError:
+                raise ValueError(f"Invalid data_type {data_type!r}")
+
+        super().__init__(*expressions, output_field=output_field)
+
+        self.key_name = key_name
+        self.data_type = data_type
 
     def as_sql(
         self, compiler: SQLCompiler, connection: BaseDatabaseWrapper
diff --git a/src/django_mysql/models/functions.py b/src/django_mysql/models/functions.py
index e5173e21..23db10c8 100644
--- a/src/django_mysql/models/functions.py
+++ b/src/django_mysql/models/functions.py
@@ -441,20 +441,16 @@ class AsType(Func):
     template = "%(expressions)s AS %(data_type)s"
 
     def __init__(self, expression: ExpressionArgument, data_type: str) -> None:
+        from django_mysql.models.fields.dynamic import KeyTransform
+
         if not hasattr(expression, "resolve_expression"):
             expression = Value(expression)
 
-        if data_type not in self.TYPE_MAP:
+        if data_type not in KeyTransform.TYPE_MAP and data_type != "BINARY":
             raise ValueError(f"Invalid data_type '{data_type}'")
 
         super().__init__(expression, data_type=data_type)
 
-    @property
-    def TYPE_MAP(self) -> dict[str, type[DjangoField] | DjangoField]:
-        from django_mysql.models.fields.dynamic import KeyTransform
-
-        return KeyTransform.TYPE_MAP
-
 
 class ColumnAdd(Func):
     function = "COLUMN_ADD"
@@ -508,25 +504,22 @@ def __init__(
         self,
         expression: ExpressionArgument,
         column_name: ExpressionArgument,
-        data_type: ExpressionArgument,
+        data_type: str,
     ):
+        from django_mysql.models.fields.dynamic import DynamicField, KeyTransform
+
         if not hasattr(column_name, "resolve_expression"):
             column_name = Value(column_name)
 
-        try:
-            output_field = self.TYPE_MAP[data_type]
-        except KeyError:
-            raise ValueError(f"Invalid data_type '{data_type}'")
-
+        output_field: DjangoField[Any, Any]
         if data_type == "BINARY":
-            output_field = output_field()
+            output_field = DynamicField()
+        else:
+            try:
+                output_field = KeyTransform.TYPE_MAP[data_type]
+            except KeyError:
+                raise ValueError(f"Invalid data_type {data_type!r}")
 
         super().__init__(
             expression, column_name, output_field=output_field, data_type=data_type
         )
-
-    @property
-    def TYPE_MAP(self) -> dict[str, DjangoField | type[DjangoField]]:
-        from django_mysql.models.fields.dynamic import KeyTransform
-
-        return KeyTransform.TYPE_MAP
diff --git a/tests/testapp/test_dynamicfield.py b/tests/testapp/test_dynamicfield.py
index 95c885b9..d324de6f 100644
--- a/tests/testapp/test_dynamicfield.py
+++ b/tests/testapp/test_dynamicfield.py
@@ -15,6 +15,7 @@
 from django.test.utils import isolate_apps
 
 from django_mysql.models import DynamicField
+from django_mysql.models.fields.dynamic import KeyTransform
 from tests.testapp.models import DynamicModel, SpeclessDynamicModel
 
 
@@ -159,6 +160,12 @@ def test_non_existent_transform(self):
     def test_has_key(self):
         assert list(DynamicModel.objects.filter(attrs__has_key="c")) == self.objs[1:3]
 
+    def test_key_transform_initialize_bad_type(self):
+        with pytest.raises(ValueError) as excinfo:
+            KeyTransform("x", "unknown")
+
+        assert str(excinfo.value) == "Invalid data_type 'unknown'"
+
     def test_key_transform_datey(self):
         assert list(DynamicModel.objects.filter(attrs__datey=dt.date(2001, 1, 4))) == [
             self.objs[4]