diff --git a/sphinx/ext/graphviz.py b/sphinx/ext/graphviz.py
index 2dabd5e7e..a5a7c92ac 100644
--- a/sphinx/ext/graphviz.py
+++ b/sphinx/ext/graphviz.py
@@ -95,7 +95,7 @@ def render_dot(self, code, options, format, prefix='graphviz'):
 
     if hasattr(self.builder, '_graphviz_warned_dot') or \
        hasattr(self.builder, '_graphviz_warned_ps2pdf'):
-        return None
+        return None, None
 
     ensuredir(path.dirname(outfn))
 
@@ -114,7 +114,7 @@ def render_dot(self, code, options, format, prefix='graphviz'):
                           'output), check the graphviz_dot setting' %
                           self.builder.config.graphviz_dot)
         self.builder._graphviz_warned_dot = True
-        return None
+        return None, None
     # graphviz expects UTF-8 by default
     if isinstance(code, unicode):
         code = code.encode('utf-8')