diff --git a/index.js b/index.js
index 70ffe78..fcba822 100644
--- a/index.js
+++ b/index.js
@@ -66,6 +66,10 @@ function hook (extension, transform, options) {
     nestedTransforms[extension].every(function (nested) {
       // nesting order performs earlier first, later last
       ret = nested(source, filename)
+      if (ret === undefined) {
+        console.error('🔥  source transform returned undefined for file',
+          filename)
+      }
       source = ret + '' // convert to string and keep going
       return true // continue
     })
diff --git a/test/jsTest.js b/test/jsTest.js
index 7677f94..bebcd0c 100644
--- a/test/jsTest.js
+++ b/test/jsTest.js
@@ -25,4 +25,4 @@ gt.test('basics', function () {
   });
   require('./dummy');
   gt.ok(transformCalled, 'transform function was called');
-});
\ No newline at end of file
+});