Fixed a bug

Cython/Coverage.py

@@ -139,7 +139,7 @@ class Plugin(CoveragePlugin):
             try:
                 with open(c_file, 'rb') as f:
                     if b'/* Generated by Cython ' not in f.read(30):
-                        return None  # not a Cython file
+                        return None, None  # not a Cython file
             except (IOError, OSError):
                 c_file = None