diff --git a/src/ca/comBuf.h b/src/ca/comBuf.h
index b74da3925..d99132a14 100644
--- a/src/ca/comBuf.h
+++ b/src/ca/comBuf.h
@@ -330,7 +330,7 @@ inline unsigned comBuf::push ( const epicsOldString * pValue, unsigned nElem )
         nBytes = nElem * sizeof ( *pValue );
     }
     memcpy ( &this->buf[ index ], pValue, nBytes );
-    this->nextWriteIndex += nBytes;
+    this->nextWriteIndex = index + nBytes;
     return nElem;
 }