diff --git a/lib/matplotlib/sphinxext/plot_directive.py b/lib/matplotlib/sphinxext/plot_directive.py
index 917f6e7578cd..8b31d08d9e3c 100644
--- a/lib/matplotlib/sphinxext/plot_directive.py
+++ b/lib/matplotlib/sphinxext/plot_directive.py
@@ -238,7 +238,7 @@ def plot_directive(name, arguments, options, content, lineno,
     Handle the plot directive.
     """
     formats = setup.config.plot_formats
-    if type(formats) == str:
+    if isinstance(formats, basestring):
         formats = eval(formats)
 
     # The user may provide a filename *or* Python code content, but not both