diff --git a/lib/compile.js b/lib/compile.js
index 2e5967f..c531343 100644
--- a/lib/compile.js
+++ b/lib/compile.js
@@ -150,6 +150,8 @@ function wrapSFXOutputs(loader, tree, outputs, sfxEntryPoints, opts) {
     var load = tree[name];
     if (load.metadata.build !== false) {
       var format = load.metadata.format;
+      if (format == 'defined')
+        return;
       // NB to be deprecated with SystemJS 0.19
       if (format == 'es6')
         format = 'esm';