diff --git a/FuzzerMutate.cpp b/FuzzerMutate.cpp
index 90d3697..9854e56 100644
--- a/FuzzerMutate.cpp
+++ b/FuzzerMutate.cpp
@@ -195,7 +195,8 @@
     Size += W.size();
   } else {  // Overwrite some bytes with W.
     if (W.size() > Size) return 0;
-    size_t Idx = UsePositionHint ? DE.GetPositionHint() : Rand(Size - W.size());
+    size_t Idx =
+        UsePositionHint ? DE.GetPositionHint() : Rand(Size + 1 - W.size());
     memcpy(Data + Idx, W.data(), W.size());
   }
   return Size;