diff --git a/lib/colocated-broccoli-plugin.js b/lib/colocated-broccoli-plugin.js
index e9a07a03..8becb3a2 100644
--- a/lib/colocated-broccoli-plugin.js
+++ b/lib/colocated-broccoli-plugin.js
@@ -102,18 +102,22 @@ module.exports = class ColocatedTemplateProcessor extends Plugin {
       let hasBackingClass = false;
       let hasTemplate = this.inputHasFile(basePath + '.hbs');
       let backingClassPath = basePath;
-
+      let ext;
       if (this.inputHasFile(basePath + '.js')) {
         backingClassPath += '.js';
+        ext = '.js';
         hasBackingClass = true;
       } else if (this.inputHasFile(basePath + '.ts')) {
         backingClassPath += '.ts';
+        ext = '.ts';
         hasBackingClass = true;
       } else if (this.inputHasFile(basePath + '.coffee')) {
         backingClassPath += '.coffee';
+        ext = '.coffee';
         hasBackingClass = true;
       } else {
         backingClassPath += '.js';
+        ext = '.js';
         hasBackingClass = false;
       }
 
@@ -173,10 +177,12 @@ module.exports = class ColocatedTemplateProcessor extends Plugin {
 
         // It's not clear to me how to format this correctly for coffeescript.
         if (!backingClassPath.endsWith('.coffee')) {
+          let file = filePathParts.name + ext;
           let jsContentsMap = jsContentsMagic.generateMap({
-            source: jsOutputPath,
-            file: jsOutputPath,
+            source: backingClassPath,
+            file,
             includeContent: true,
+            hires: true
           });
 
           jsContents += `\n//# sourceMappingURL=${jsContentsMap.toUrl()}`;