diff --git a/lib/source-package.js b/lib/source-package.js
index 661c06a..1db63d3 100644
--- a/lib/source-package.js
+++ b/lib/source-package.js
@@ -96,7 +96,7 @@ var path              = require('path')
           this.packageJSON = packageJSON
 
           if (!this.packageJSON.main) this.packageJSON.main = './index.js'
-          if (!this.packageJSON.main.match(/\.js$/)) this.packageJSON.main += '.js'
+          if (typeof this.packageJSON.main == "string" &&  !this.packageJSON.main.match(/\.js$/)) this.packageJSON.main += '.js'
 
           // custom hasher function for async.memoize so we have a single key, default will use
           // first arg (callback) as hash key which won't work
@@ -196,4 +196,4 @@ var path              = require('path')
 
 module.exports.create = function (packageName, parents, isClient, packageJSON, options) {
   return Object.create(SourcePackage).init(packageName, parents, isClient, packageJSON, options)
-}
\ No newline at end of file
+}