diff --git a/lib/Rewrite/RewriteRope.cpp b/lib/Rewrite/RewriteRope.cpp
index 0a4dc85..61cb02b 100644
--- a/lib/Rewrite/RewriteRope.cpp
+++ b/lib/Rewrite/RewriteRope.cpp
@@ -792,7 +792,7 @@
   if (AllocBuffer && --AllocBuffer->RefCount == 0)
     delete [] (char*)AllocBuffer;
   
-  unsigned AllocSize = sizeof(RopeRefCountString)-1+AllocChunkSize;
+  unsigned AllocSize = offsetof(RopeRefCountString, Data) + AllocChunkSize;
   AllocBuffer = reinterpret_cast<RopeRefCountString *>(new char[AllocSize]);
   AllocBuffer->RefCount = 0;
   memcpy(AllocBuffer->Data, Start, Len);