--- a/python/mozbuild/mozbuild/test/test_preprocessor.py
+++ b/python/mozbuild/mozbuild/test/test_preprocessor.py
@@ -614,23 +614,23 @@ class TestPreprocessor(unittest.TestCase
                               '//@line 6 "CWD/f.js"\n'
                               'fin\n').replace('CWD/',
                                                os.getcwd() + os.path.sep))
 
     def test_include_missing_file(self):
         with MockedOpen({'f': '#include foo\n'}):
             with self.assertRaises(Preprocessor.Error) as e:
                 self.pp.do_include('f')
-                self.assertEqual(e.key, 'FILE_NOT_FOUND')
+            self.assertEqual(e.exception.key, 'FILE_NOT_FOUND')
 
     def test_include_undefined_variable(self):
         with MockedOpen({'f': '#filter substitution\n#include @foo@\n'}):
             with self.assertRaises(Preprocessor.Error) as e:
                 self.pp.do_include('f')
-                self.assertEqual(e.key, 'UNDEFINED_VAR')
+            self.assertEqual(e.exception.key, 'UNDEFINED_VAR')
 
     def test_include_literal_at(self):
         files = {
             '@foo@': '#define foo foobarbaz\n',
             'f': '#include @foo@\n#filter substitution\n@foo@\n',
         }
 
         with MockedOpen(files):