diff --git a/lib/sourcemap.js b/lib/sourcemap.js
index c2587c88..8e4f6931 100644
--- a/lib/sourcemap.js
+++ b/lib/sourcemap.js
@@ -144,7 +144,9 @@ function SourceMap(options) {
             add(source, gen_line, gen_col, orig_line, orig_col, name);
         } : add,
         setSourceContent: sources_content ? function(source, content) {
-            sources_content[source] = content;
+            if (!sources_content[source]) {
+                sources_content[source] = content;
+            }
         } : noop,
         toString: function() {
             return JSON.stringify({