diff --git a/pygal/graph/base.py b/pygal/graph/base.py
index 1bccf3e..639e20f 100644
--- a/pygal/graph/base.py
+++ b/pygal/graph/base.py
@@ -205,8 +205,8 @@ class BaseGraph(object):
 
     def render_to_png(self, filename):
         import cairosvg
-        import StringIO
-        fakefile = StringIO.StringIO()
+        from io import BytesIO
+        fakefile = BytesIO()
         fakefile.write(self.render())
         fakefile.seek(0)
         cairosvg.surface.PNGSurface.convert(
diff --git a/pygal/test/test_graph.py b/pygal/test/test_graph.py
index 2dcda95..dc43e02 100644
--- a/pygal/test/test_graph.py
+++ b/pygal/test/test_graph.py
@@ -61,6 +61,6 @@ def test_render_to_png():
     line = Line()
     line.add('Serie 1', [1])
     line.render_to_png(file_name)
-    with open(file_name) as f:
+    with open(file_name, 'rb') as f:
         assert f.read()
     os.remove(file_name)