diff --git a/src/cmsplugin_blurp/renderers/data_source.py b/src/cmsplugin_blurp/renderers/data_source.py
index 454c4e2..e61d448 100644
--- a/src/cmsplugin_blurp/renderers/data_source.py
+++ b/src/cmsplugin_blurp/renderers/data_source.py
@@ -74,7 +74,7 @@ class Renderer(template.TemplateRenderer):
                    yield 'unknown auth_mech {0!r}'.format(source['auth_mech'])
                if     source['auth_mech'].startswith('hmac-') \
                   and ('signature_key' not in source
-                       or not isinstance(source['signature_key'])):
+                       or not isinstance(source['signature_key'], basestring)):
                    yield 'missing signature_key string'
 
     def get_sources(self, context):