diff --git a/lib/sourcemap.js b/lib/sourcemap.js
index 232cfabda9a..1fdf15c5a40 100644
--- a/lib/sourcemap.js
+++ b/lib/sourcemap.js
@@ -100,21 +100,29 @@ function SourceMap(options) {
     var sources_content = options.content && Object.create(null);
     var names = create_array_map();
     var mappings = "";
-    if (options.orig) Object.keys(options.orig).forEach(function(name) {
-        var map = options.orig[name];
-        var indices = [ 0, 0, 1, 0, 0 ];
-        map.mappings = map.mappings.split(/;/).map(function(line) {
-            indices[0] = 0;
-            return line.split(/,/).map(function(segment) {
-                return indices.slice(0, vlq_decode(indices, segment));
-            });
+    var orig_maps;
+    if (options.orig) {
+        orig_maps = Object.create(null);
+        Object.keys(options.orig).forEach(function(name) {
+            var map = options.orig[name];
+            var indices = [ 0, 0, 1, 0, 0 ];
+            orig_maps[name] = {
+                names: map.names,
+                mappings: map.mappings.split(/;/).map(function(line) {
+                    indices[0] = 0;
+                    return line.split(/,/).map(function(segment) {
+                        return indices.slice(0, vlq_decode(indices, segment));
+                    });
+                }),
+                sources: map.sources,
+            };
+            if (!sources_content || !map.sourcesContent) return;
+            for (var i = 0; i < map.sources.length; i++) {
+                var content = map.sourcesContent[i];
+                if (content) sources_content[map.sources[i]] = content;
+            }
         });
-        if (!sources_content || !map.sourcesContent) return;
-        for (var i = 0; i < map.sources.length; i++) {
-            var content = map.sourcesContent[i];
-            if (content) sources_content[map.sources[i]] = content;
-        }
-    });
+    }
     var generated_line = 1;
     var generated_column = 0;
     var source_index = 0;
@@ -122,8 +130,8 @@ function SourceMap(options) {
     var original_column = 0;
     var name_index = 0;
     return {
-        add: options.orig ? function(source, gen_line, gen_col, orig_line, orig_col, name) {
-            var map = options.orig[source];
+        add: orig_maps ? function(source, gen_line, gen_col, orig_line, orig_col, name) {
+            var map = orig_maps[source];
             if (map) {
                 var segments = map.mappings[orig_line - 1];
                 if (!segments) return;
diff --git a/test/mocha/sourcemaps.js b/test/mocha/sourcemaps.js
index f8e399ce585..23698c575d7 100644
--- a/test/mocha/sourcemaps.js
+++ b/test/mocha/sourcemaps.js
@@ -277,6 +277,12 @@ describe("sourcemaps", function() {
     });
 
     describe("input sourcemaps", function() {
+        it("Should not modify input source map", function() {
+            var orig = get_map();
+            var original = JSON.stringify(orig);
+            var map = prepare_map(orig);
+            assert.strictEqual(JSON.stringify(orig), original);
+        });
         it("Should copy over original sourcesContent", function() {
             var orig = get_map();
             var map = prepare_map(orig);