diff --git a/mozilla/extensions/transformiix/source/xpath/StringFunctionCall.cpp b/mozilla/extensions/transformiix/source/xpath/StringFunctionCall.cpp
index 74dd2cc363e..c68da321f8a 100644
--- a/mozilla/extensions/transformiix/source/xpath/StringFunctionCall.cpp
+++ b/mozilla/extensions/transformiix/source/xpath/StringFunctionCall.cpp
@@ -169,7 +169,7 @@ ExprResult* StringFunctionCall::evaluate(Node* aContext, ContextState* aCs)
             // check for NaN or +/-Inf
             if (Double::isNaN(start) ||
                 Double::isInfinite(start) ||
-                start + 0.5 >= src.length())
+                start >= src.length() + 0.5)
                 return new StringResult();
 
             start = floor(start + 0.5) - 1;