--- a/build/moz.configure/checks.configure
+++ b/build/moz.configure/checks.configure
@@ -70,10 +70,11 @@ def check_prog(var, progs, allow_missing
     @depends(check)
     @advanced
     def postcheck(value):
         set_config(var, ':' if value is not_found else value)
         if value is not_found and not allow_missing:
             from mozbuild.shellutil import quote
             error('Cannot find %s (tried: %s)'
                   % (var.lower(), ', '.join(quote(p) for p in progs)))
+        return None if value is not_found else value
 
-    return check
+    return postcheck