diff --git a/lib/minify.js b/lib/minify.js
index d84f6825..0da92ac4 100644
--- a/lib/minify.js
+++ b/lib/minify.js
@@ -111,6 +111,7 @@ function minify(files, options) {
                 content: null,
                 filename: null,
                 includeSources: false,
+                json: true,
                 root: null,
                 url: null,
             }, true);
@@ -207,7 +208,13 @@ function minify(files, options) {
             toplevel.print(stream);
             result.code = stream.get();
             if (options.sourceMap) {
-                result.map = options.output.source_map.toString();
+                result.map = options.output.source_map.get().toJSON();
+                if (Array.isArray(files)) {
+                    result.map.sources = [null];
+                }
+                if (options.sourceMap.json !== false) {
+                    result.map = JSON.stringify(result.map);
+                }
                 if (options.sourceMap.url == "inline") {
                     result.code += "\n//# sourceMappingURL=data:application/json;charset=utf-8;base64," + to_base64(result.map);
                 } else if (options.sourceMap.url) {