diff --git a/IPython/nbconvert/tests/base.py b/IPython/nbconvert/tests/base.py
index 26f6ec6..81df37b 100644
--- a/IPython/nbconvert/tests/base.py
+++ b/IPython/nbconvert/tests/base.py
@@ -150,6 +150,6 @@ class TestsBase(ParametricTestCase):
         """
 
         stdout, stderr, retcode = get_output_error_code(ipy_cmd + parameters)
-        if retcode != 0 and not ignore_return_code:
+        if not (retcode == 0 or ignore_return_code):
             raise OSError(stderr)
         return stdout, stderr