diff --git a/openapi_core/schema/schemas/util.py b/openapi_core/schema/schemas/util.py
index cf2c917..4a87f46 100644
--- a/openapi_core/schema/schemas/util.py
+++ b/openapi_core/schema/schemas/util.py
@@ -1,9 +1,10 @@
 """OpenAPI core schemas util module"""
 from distutils.util import strtobool
+from six import string_types
 
 
 def forcebool(val):
-    if isinstance(val, str):
+    if isinstance(val, string_types):
         val = strtobool(val)
 
     return bool(val)