diff --git a/lib/processPbrMetallicRoughness.js b/lib/processPbrMetallicRoughness.js
index b071a4f8..31349d59 100644
--- a/lib/processPbrMetallicRoughness.js
+++ b/lib/processPbrMetallicRoughness.js
@@ -595,12 +595,8 @@ function generateTechnique(gltf, material, options) {
     var alphaMode = material.alphaMode;
     if (defined(alphaMode)) {
         if (alphaMode === 'MASK') {
-            var alphaCutoff = material.alphaCutoff;
-            if (defined(alphaCutoff)) {
-                fragmentShader += '    gl_FragColor = vec4(color, int(baseColorWithAlpha.a >= ' + alphaCutoff + '));\n';
-            } else {
-                fragmentShader += '    gl_FragColor = vec4(color, 1.0);\n';
-            }
+            var alphaCutoff = defaultValue(material.alphaCutoff, 0.5);
+            fragmentShader += '    gl_FragColor = vec4(color, int(baseColorWithAlpha.a >= ' + alphaCutoff + '));\n';
         } else if (alphaMode === 'BLEND') {
             fragmentShader += '    gl_FragColor = vec4(color, baseColorWithAlpha.a);\n';
         } else {