diff --git a/lib/ace/ext/whitespace.js b/lib/ace/ext/whitespace.js
index cbbdbb43..583464ab 100644
--- a/lib/ace/ext/whitespace.js
+++ b/lib/ace/ext/whitespace.js
@@ -61,7 +61,7 @@ exports.$detectIndentation = function(lines, fallback) {
         prevSpaces = spaces;
 
         // ignore lines ending with backslash
-        while (line[line.length - 1] == "\\")
+        while (i < max && line[line.length - 1] == "\\")
             line = lines[i++];
     }