diff --git a/wagtail/tests/models.py b/wagtail/tests/models.py
index 66709b897..53a9b2796 100644
--- a/wagtail/tests/models.py
+++ b/wagtail/tests/models.py
@@ -404,7 +404,7 @@ class BusinessSubIndex(Page):
 class BusinessChild(Page):
     """ Can only be placed under Business indexes, no children allowed """
     subpage_types = []
-    parent_page_types = ['tests.BusinessIndex', 'tests.BusinessSubIndex']
+    parent_page_types = ['tests.BusinessIndex', BusinessSubIndex]
 
 
 class SearchTest(models.Model, index.Indexed):
diff --git a/wagtail/wagtailcore/utils.py b/wagtail/wagtailcore/utils.py
index dcd3cf6d5..1880684bc 100644
--- a/wagtail/wagtailcore/utils.py
+++ b/wagtail/wagtailcore/utils.py
@@ -31,8 +31,8 @@ def resolve_model_string(model_string, default_app=None):
             raise LookupError("Can not resolve {0!r} into a model".format(model_string), model_string)
         return model
 
-    elif model_string is not None and issubclass(model_string, Model):
-        return model
+    elif isinstance(model_string, type) and issubclass(model_string, Model):
+        return model_string
 
     else:
         raise LookupError("Can not resolve {0!r} into a model".format(model_string), model_string)