make sure we do not fail if OpenLayers.Projection.defaults has not been set for a custom projection

lib/OpenLayers/Layer/WMS.js

@@ -177,7 +177,8 @@ OpenLayers.Layer.WMS = OpenLayers.Class(OpenLayers.Layer.Grid, {
     reverseAxisOrder: function() {
         var projCode = this.projection.getCode();
         return parseFloat(this.params.VERSION) >= 1.3 && 
-            !!(this.yx[projCode] || OpenLayers.Projection.defaults[projCode].yx);
+            !!(this.yx[projCode] || (OpenLayers.Projection.defaults[projCode] && 
+            OpenLayers.Projection.defaults[projCode].yx));
     },
     
     /**

tests/Layer/WMS.html

@@ -423,6 +423,26 @@
         t.ok( layer.grid == null, "grid set to null");
     }
 
+    function test_customProjection(t) {
+        t.plan(1);
+        var map = new OpenLayers.Map('map', {
+            units: 'm', 
+            projection: new OpenLayers.Projection('EPSG:28992'),
+            maxExtent: new OpenLayers.Bounds(0, 300000, 300000, 6250000)
+        });
+        var layer = new OpenLayers.Layer.WMS(null, url, {layers: 'mylayer', version: '1.3.0'});
+        map.addLayer(layer);
+        var error = false;
+        try {
+            map.setCenter(new OpenLayers.LonLat(100000,300000), 5);
+        } catch(err) {
+            error = true;
+        }
+        t.ok(!error, "no error on getURL if layer has a custom projection and no defaults defined");
+        layer.destroy();
+        map.destroy();
+    }
+
     function test_Layer_WMS_v13(t) {
 
         t.plan(6);