--- a/python/mozbuild/mozbuild/backend/tup.py
+++ b/python/mozbuild/mozbuild/backend/tup.py
@@ -124,18 +124,18 @@ class BackendTupfile(object):
             ('.c', 'CC', 'CFLAGS'),
         ]
         for extension, compiler, flags in compilers:
             srcs = sorted(self.sources[extension])
             for src in srcs:
                 # AS can be set to $(CC), so we need to call expand_variables on
                 # the compiler to get the real value.
                 cmd = [expand_variables(self.environment.substs[compiler], self.environment.substs)]
-                cmd.extend(self.local_flags[flags])
-                cmd.extend(self.per_source_flags[src])
+                cmd.extend(shell_quote(f) for f in self.local_flags[flags])
+                cmd.extend(shell_quote(f) for f in self.per_source_flags[src])
                 cmd.extend(['-c', '%f', '-o', '%o'])
                 self.rule(
                     cmd=cmd,
                     inputs=[src],
                     extra_inputs=extra_inputs,
                     outputs=['%B.o'],
                     display='%s %%f' % compiler,
                 )