diff --git a/pytest_cases/fixture_parametrize_plus.py b/pytest_cases/fixture_parametrize_plus.py
index fbaed1a6..df19b84b 100644
--- a/pytest_cases/fixture_parametrize_plus.py
+++ b/pytest_cases/fixture_parametrize_plus.py
@@ -32,7 +32,7 @@
 from .common_pytest import get_fixture_name, remove_duplicates, mini_idvalset, is_marked_parameter_value, \
     extract_parameterset_info, ParameterSet, cart_product_pytest, mini_idval, inject_host, \
     get_marked_parameter_values, resolve_ids, get_marked_parameter_id, get_marked_parameter_marks, is_fixture, \
-    safe_isinstance
+    safe_isclass
 
 from .fixture__creation import check_name_available, CHANGE, WARN
 from .fixture_core1_unions import InvalidParamsList, NOT_USED, UnionFixtureAlternative, _make_fixture_union, \
@@ -789,7 +789,7 @@ def _make_ids(**args):
         else:
             # wrap the decorator to check if the test function has the parameters as arguments
             def _apply(test_func):
-                if not safe_isinstance(test_func, type):
+                if not safe_isclass(test_func):
                     # a Function: raise a proper error message if improper use
                     s = signature(test_func)
                     for p in argnames:
@@ -932,7 +932,7 @@ def parametrize_plus_decorate(test_func, fixtures_dest):
             test_func_name = test_func.__name__
 
             # first check if the test function has the parameters as arguments
-            if safe_isinstance(test_func, type):
+            if safe_isclass(test_func):
                 # a test class: not supported yet
                 raise NotImplementedError("@parametrize can not be used to decorate a Test class when the argvalues "
                                           "contain at least one reference to a fixture.")