diff --git a/mozilla/js/tests/jsDriver.pl b/mozilla/js/tests/jsDriver.pl
index 3cfe634d56e..73c27ea77d8 100755
--- a/mozilla/js/tests/jsDriver.pl
+++ b/mozilla/js/tests/jsDriver.pl
@@ -606,6 +606,11 @@ sub get_xpc_engine_command {
         $m5_home .= "/";
     }
 
+    if (($os_type ne "MAC") && !(-x &xp_path($m5_home . "xpcshell"))) {
+        # mac doesn't seem to deal with -x correctly
+        die (&xp_path($m5_home . "xpcshell") . " is not a valid executable on this system.\n");
+    }
+
     return &xp_path($m5_home . "xpcshell");
 
 }
@@ -739,7 +744,7 @@ sub get_dd_engine_command {
         }
 
 
-        if (($os_type ne "MAC") && !(-x $retval)) {
+        if (($os_type ne "MAC") && !(-x &xp_path($retval))) {
             # mac doesn't seem to deal with -x correctly
             die ($retval . " is not a valid executable on this system.\n");
         }