diff --git a/lib/corereqs.js b/lib/corereqs.js
index 5a93de0..f7ecb91 100644
--- a/lib/corereqs.js
+++ b/lib/corereqs.js
@@ -213,13 +213,16 @@ function packValueMask(reqname, values)
     if (!reqValueMask)
         throw new Error(reqname + ': no value mask description');
 
-    for (var v in values)
+    for (var value in values)
     {
-        var valueBit = reqValueMask[v].mask;
-        if (!valueBit)
-            throw new Error(reqname + ': incorrect value param ' + v);
-        masksList.push(valueBit);
-        bitmask |= valueBit;
+        var v = reqValueMask[value];
+        if (v) {
+          var valueBit = v.mask;
+          if (!valueBit)
+              throw new Error(reqname + ': incorrect value param ' + value);
+          masksList.push(valueBit);
+          bitmask |= valueBit;
+        }
     }
 
     /* numeric sort */
diff --git a/test/configure-window.js b/test/configure-window.js
index 00ec6ba..ec03103 100644
--- a/test/configure-window.js
+++ b/test/configure-window.js
@@ -82,7 +82,19 @@ describe('ConfigureWindow', function() {
         this.X.LowerWindow(this.wid);
     });
 
+    it('should ignore invalid mask values', function(done) {
+        this.X.once('event', function(ev) {
+            ev.x.should.equal(0);
+            done();
+        });
+
+        this.X.ConfigureWindow(this.wid, { foo : 3, x : 0 }, function(err) {
+            console.log(err);
+        });
+    });
+
     after(function(done) {
+        this.X.removeAllListeners('event');
         this.X.DestroyWindow(this.wid);
         this.X.DestroyWindow(this.wid_helper);
         this.X.on('end', done);