diff --git a/lib/less/source-map-output.js b/lib/less/source-map-output.js
index 5692e6b40..8659535e7 100644
--- a/lib/less/source-map-output.js
+++ b/lib/less/source-map-output.js
@@ -69,6 +69,12 @@ module.exports = function (environment) {
                 // adjust the source
                 inputSource = inputSource.slice(this._contentsIgnoredCharsMap[fileInfo.filename]);
             }
+
+            // ignore empty content
+            if (inputSource === undefined) {
+                return;
+            }
+
             inputSource = inputSource.substring(0, index);
             sourceLines = inputSource.split('\n');
             sourceColumns = sourceLines[sourceLines.length - 1];