diff --git a/lib/output.js b/lib/output.js
index dceece34..2786941e 100644
--- a/lib/output.js
+++ b/lib/output.js
@@ -308,7 +308,7 @@ function OutputStream(options) {
     var add_mapping = options.source_map ? function(token, name) {
         try {
             if (token) options.source_map.add(
-                token.file || "?",
+                token.file,
                 current_line, current_col,
                 token.line, token.col,
                 (!name && token.type == "name") ? token.value : name
diff --git a/lib/sourcemap.js b/lib/sourcemap.js
index a67011f0..f62b43e1 100644
--- a/lib/sourcemap.js
+++ b/lib/sourcemap.js
@@ -53,19 +53,33 @@ function SourceMap(options) {
         orig_line_diff : 0,
         dest_line_diff : 0,
     });
+    var orig_maps = Object.create(null);
     var orig_map = options.orig && new MOZ_SourceMap.SourceMapConsumer(options.orig);
     var generator;
     if (orig_map) {
-      generator = MOZ_SourceMap.SourceMapGenerator.fromSourceMap(orig_map);
+        generator = MOZ_SourceMap.SourceMapGenerator.fromSourceMap(orig_map);
     } else {
         generator = new MOZ_SourceMap.SourceMapGenerator({
             file       : options.file,
             sourceRoot : options.root
         });
     }
+    function addInput(rawSourceMap) {
+        var consumer = new MOZ_SourceMap.SourceMapConsumer(rawSourceMap);
+        orig_maps[consumer.file] = consumer;
+    }
     function add(source, gen_line, gen_col, orig_line, orig_col, name) {
-        if (orig_map) {
-            var info = orig_map.originalPositionFor({
+        var originalMap;
+        if (source) {
+            originalMap = orig_maps[source] || orig_map;
+        }
+        else {
+            source = "?";
+            originalMap = orig_map;
+        }
+
+        if (originalMap) {
+            var info = originalMap.originalPositionFor({
                 line: orig_line,
                 column: orig_col
             });
@@ -77,6 +91,7 @@ function SourceMap(options) {
             orig_col = info.column;
             name = info.name || name;
         }
+
         generator.addMapping({
             generated : { line: gen_line + options.dest_line_diff, column: gen_col },
             original  : { line: orig_line + options.orig_line_diff, column: orig_col },
@@ -85,6 +100,7 @@ function SourceMap(options) {
         });
     }
     return {
+        addInput   : addInput,
         add        : add,
         get        : function() { return generator },
         toString   : function() { return JSON.stringify(generator.toJSON()); }