diff --git a/lib/codemirror.js b/lib/codemirror.js
index 969302a2fc..ebaa643cbe 100644
--- a/lib/codemirror.js
+++ b/lib/codemirror.js
@@ -878,7 +878,7 @@ var CodeMirror = (function() {
         if (n) indentation = lines[n-1].indentation();
         else indentation = 0;
       }
-      else if (how == "smart") indentation = mode.indent(state, line.text.slice(curSpace));
+      else if (how == "smart") indentation = mode.indent(state, line.text.slice(curSpaceString.length));
       else if (how == "add") indentation = curSpace + options.indentUnit;
       else if (how == "subtract") indentation = Math.max(0, curSpace - options.indentUnit);
       var diff = indentation - curSpace;