diff --git a/source/blender/python/generic/py_capi_utils.c b/source/blender/python/generic/py_capi_utils.c
index a2521484c88..2b4dcf93b66 100644
--- a/source/blender/python/generic/py_capi_utils.c
+++ b/source/blender/python/generic/py_capi_utils.c
@@ -760,7 +760,6 @@ int PyC_FlagSet_ValueFromID(PyC_FlagSet *item, const char *identifier, int *valu
 	return 0;
 }
 
-/* 'value' _must_ be a set type, error check before calling */
 int PyC_FlagSet_ToBitfield(PyC_FlagSet *items, PyObject *value, int *r_value, const char *error_prefix)
 {
 	/* set of enum items, concatenate all values with OR */
@@ -771,6 +770,15 @@ int PyC_FlagSet_ToBitfield(PyC_FlagSet *items, PyObject *value, int *r_value, co
 	Py_ssize_t hash = 0;
 	PyObject *key;
 
+	PyC_ObSpit("", value);
+
+	if (!PySet_Check(value)) {
+		PyErr_Format(PyExc_TypeError,
+		             "%.200s expected a set, not %.200s",
+		             error_prefix, Py_TYPE(value)->tp_name);
+		return -1;
+	}
+
 	*r_value = 0;
 
 	while (_PySet_NextEntry(value, &pos, &key, &hash)) {
@@ -778,7 +786,7 @@ int PyC_FlagSet_ToBitfield(PyC_FlagSet *items, PyObject *value, int *r_value, co
 
 		if (param == NULL) {
 			PyErr_Format(PyExc_TypeError,
-			             "%.200s expected a string, not %.200s",
+			             "%.200s set must contain strings, not %.200s",
 			             error_prefix, Py_TYPE(key)->tp_name);
 			return -1;
 		}