diff -r 4393b7db103b -r add89df8447e rt/mojo/src/main/java/org/apidesign/bck2brwsr/mojo/Java2JavaScript.java
--- a/rt/mojo/src/main/java/org/apidesign/bck2brwsr/mojo/Java2JavaScript.java	Thu Oct 10 13:59:29 2013 +0200
+++ b/rt/mojo/src/main/java/org/apidesign/bck2brwsr/mojo/Java2JavaScript.java	Thu Oct 10 14:00:32 2013 +0200
@@ -80,6 +80,9 @@
         if (!classes.isDirectory()) {
             throw new MojoExecutionException("Can't find " + classes);
         }
+        if (javascript == null) {
+            throw new MojoExecutionException("Need to define 'javascript' attribute with a path to file to generate");
+        }
 
         List<String> arr = new ArrayList<String>();
         long newest = collectAllClasses("", classes, arr);