diff --git a/lib/build.js b/lib/build.js
index 81c2af8..4a6a03c 100644
--- a/lib/build.js
+++ b/lib/build.js
@@ -191,7 +191,7 @@ function getWebpackOpts(opts, callback) {
     }));
   }
 
-  if(opts.build.define){
+  if(opts.build.define && typeof(opts.build.define) === 'string'){
 
     var pkgDefine = {};