diff --git a/lib/sourcemap.js b/lib/sourcemap.js
index 663ef12e..7b74bd23 100644
--- a/lib/sourcemap.js
+++ b/lib/sourcemap.js
@@ -53,10 +53,7 @@ function SourceMap(options) {
         orig_line_diff : 0,
         dest_line_diff : 0,
     });
-    var generator = new MOZ_SourceMap.SourceMapGenerator({
-        file       : options.file,
-        sourceRoot : options.root
-    });
+    var generator;
     var orig_map = options.orig && new MOZ_SourceMap.SourceMapConsumer(options.orig);
     function add(source, gen_line, gen_col, orig_line, orig_col, name) {
         if (orig_map) {
@@ -78,7 +75,15 @@ function SourceMap(options) {
             source    : source,
             name      : name
         });
-    };
+    }
+    if (orig_map) {
+      generator = MOZ_SourceMap.SourceMapGenerator.fromSourceMap(orig_map);
+    } else {
+      generator = new MOZ_SourceMap.SourceMapGenerator({
+          file       : options.file,
+          sourceRoot : options.root
+      });
+    }
     return {
         add        : add,
         get        : function() { return generator },