diff --git a/Tests/test_imagecms.py b/Tests/test_imagecms.py
index 6be29a70f..c04d08806 100644
--- a/Tests/test_imagecms.py
+++ b/Tests/test_imagecms.py
@@ -506,6 +506,12 @@ def test_profile_typesafety() -> None:
     with pytest.raises(TypeError, match="Invalid type for Profile"):
         ImageCms.ImageCmsProfile(1).tobytes()
 
+    # also check core function
+    with pytest.raises(TypeError):
+        ImageCms.core.profile_tobytes(0)
+    with pytest.raises(TypeError):
+        ImageCms.core.profile_tobytes(1)
+
 
 def assert_aux_channel_preserved(
     mode: str, transform_in_place: bool, preserved_channel: str
diff --git a/src/_imagingcms.c b/src/_imagingcms.c
index c7728770a..7c7763653 100644
--- a/src/_imagingcms.c
+++ b/src/_imagingcms.c
@@ -143,7 +143,7 @@ cms_profile_tobytes(PyObject *self, PyObject *args) {
     cmsHPROFILE *profile;
 
     PyObject *ret;
-    if (!PyArg_ParseTuple(args, "O", &CmsProfile)) {
+    if (!PyArg_ParseTuple(args, "O!", &CmsProfile_Type, &CmsProfile)) {
         return NULL;
     }