diff --git a/IPython/nbconvert/transformers/tests/base.py b/IPython/nbconvert/transformers/tests/base.py
index 1d2a63d..56fb249 100644
--- a/IPython/nbconvert/transformers/tests/base.py
+++ b/IPython/nbconvert/transformers/tests/base.py
@@ -36,7 +36,7 @@ class TransformerTestsBase(TestsBase):
                    nbformat.new_output(output_type="stream", stream="stdout", output_text="d"),
                    nbformat.new_output(output_type="stream", stream="stderr", output_text="e"),
                    nbformat.new_output(output_type="stream", stream="stderr", output_text="f"),
-                   nbformat.new_output(output_type="png", output_png=u'Zw==')] #g
+                   nbformat.new_output(output_type="png", output_png=b'Zw==')] #g
         
         cells=[nbformat.new_code_cell(input="$ e $", prompt_number=1,outputs=outputs),
                nbformat.new_text_cell('markdown', source="$ e $")]