diff --git a/lib/sourcemap.js b/lib/sourcemap.js
index cc681026..94966a6a 100644
--- a/lib/sourcemap.js
+++ b/lib/sourcemap.js
@@ -144,7 +144,7 @@ function SourceMap(options) {
             add(source, gen_line, gen_col, orig_line, orig_col, name);
         } : add,
         setSourceContent: sources_content ? function(source, content) {
-            if (!HOP(sources_content, source)) {
+            if (!(source in sources_content)) {
                 sources_content[source] = content;
             }
         } : noop,