diff --git a/py/compat/doctest.py b/py/compat/doctest.py
index e4348b525..7e48f7358 100644
--- a/py/compat/doctest.py
+++ b/py/compat/doctest.py
@@ -1324,7 +1324,7 @@ class DocTestRunner:
     __LINECACHE_FILENAME_RE = re.compile(r'<doctest '
                                          r'(?P<name>[\w\.]+)'
                                          r'\[(?P<examplenum>\d+)\]>$')
-    def __patched_linecache_getlines(self, filename):
+    def __patched_linecache_getlines(self, filename, additional_arg=None):
         m = self.__LINECACHE_FILENAME_RE.match(filename)
         if m and m.group('name') == self.test.name:
             example = self.test.examples[int(m.group('examplenum'))]