diff --git a/common/rdr/TLSInStream.cxx b/common/rdr/TLSInStream.cxx
index 4d2c9ec..21b6056 100644
--- a/common/rdr/TLSInStream.cxx
+++ b/common/rdr/TLSInStream.cxx
@@ -44,7 +44,7 @@
       return -1;
     }
 
-    if (in->getend() - in->getptr() < size)
+    if (in->getend() - in->getptr() < (ptrdiff_t)size)
       size = in->getend() - in->getptr();
   
     in->readBytes(data, size);