diff --git a/lib/autoprefix-processor.js b/lib/autoprefix-processor.js
index debe7b0..73a5b42 100644
--- a/lib/autoprefix-processor.js
+++ b/lib/autoprefix-processor.js
@@ -38,7 +38,7 @@ module.exports = function(less) {
             var processed = postcss([autoprefixer(options)]).process(css, processOptions);
 
             if (sourceMap && !sourceMapInline) {
-                sourceMap.setExternalSourceMap(processed.map);
+                sourceMap.setExternalSourceMap(processed.map.toString());
             }
 
             return processed.css;