diff --git a/src/client/src/tscFunctionImpl.c b/src/client/src/tscFunctionImpl.c
index e02e0b3cebca14edf5068eaa4be92209decbb7b3..248b197d5240e2ee0c680e46f4c542ccce21446e 100644
--- a/src/client/src/tscFunctionImpl.c
+++ b/src/client/src/tscFunctionImpl.c
@@ -3194,7 +3194,7 @@ static void diff_function(SQLFunctionCtx *pCtx) {
     GET_RES_INFO(pCtx)->numOfRes += forwardStep;
 
     pCtx->aOutputBuf += forwardStep * pCtx->outputBytes;
-    pCtx->ptsOutputBuf += forwardStep * TSDB_KEYSIZE;
+    pCtx->ptsOutputBuf = (char*)pCtx->ptsOutputBuf + forwardStep * TSDB_KEYSIZE;
   }
 }