--- a/python/mozbuild/mozbuild/test/configure/test_toolchain_helpers.py
+++ b/python/mozbuild/mozbuild/test/configure/test_toolchain_helpers.py
@@ -185,16 +185,20 @@ class FakeCompiler(dict):
                     apply_defn(defn)
 
             for flag in flags:
                 apply_defn(self.get(flag, {}))
 
             pp.out = StringIO()
             pp.do_include(file)
             return 0, pp.out.getvalue(), ''
+        elif '-c' in flags:
+            if '-funknown-flag' in flags:
+                return 1, '', ''
+            return 0, '', ''
 
         return 1, '', ''
 
     def __add__(self, other):
         return FakeCompiler(self, other)
 
 
 class TestFakeCompiler(unittest.TestCase):