diff --git a/engine/src/core-data/Common/MatDefs/Light/Lighting.frag b/engine/src/core-data/Common/MatDefs/Light/Lighting.frag
index 8d5c785e0..8b8cc827e 100644
--- a/engine/src/core-data/Common/MatDefs/Light/Lighting.frag
+++ b/engine/src/core-data/Common/MatDefs/Light/Lighting.frag
@@ -188,20 +188,22 @@ void main(){
            specularColor.rgb *= texture2D(m_ColorRamp, vec2(light.y, 0.0)).rgb;
        #endif
 
+       // Workaround, since it is not possible to modify varying variables
+       vec4 SpecularSum2 = SpecularSum;
        #ifdef USE_REFLECTION
             vec4 refColor = Optics_GetEnvColor(m_EnvMap, refVec.xyz);
 
             // Interpolate light specularity toward reflection color
             // Multiply result by specular map
-            specularColor = mix(SpecularSum * light.y, refColor, refVec.w) * specularColor;
+            specularColor = mix(SpecularSum2 * light.y, refColor, refVec.w) * specularColor;
 
-            SpecularSum = vec4(1.0);
+            SpecularSum2 = vec4(1.0);
             light.y = 1.0;
        #endif
 
        gl_FragColor =  AmbientSum * diffuseColor +
                        DiffuseSum * diffuseColor  * light.x +
-                       SpecularSum * specularColor * light.y;
+                       SpecularSum2 * specularColor * light.y;
     #endif
     gl_FragColor.a = alpha;
 }
diff --git a/engine/src/core-plugins/com/jme3/scene/plugins/OBJLoader.java b/engine/src/core-plugins/com/jme3/scene/plugins/OBJLoader.java
index 99202d4b3..58a13f819 100644
--- a/engine/src/core-plugins/com/jme3/scene/plugins/OBJLoader.java
+++ b/engine/src/core-plugins/com/jme3/scene/plugins/OBJLoader.java
@@ -332,6 +332,9 @@ public final class OBJLoader implements AssetLoader {
         }else if (cmd.equals("usemtl")){
             // use material from MTL lib for the following faces
             currentMatName = scan.next();
+            if (!matList.containsKey(currentMatName))
+                throw new IOException("Cannot locate material " + currentMatName + " in MTL file!");
+            
         }else if (cmd.equals("mtllib")){
             // specify MTL lib to use for this OBJ file
             String mtllib = scan.nextLine().trim();