diff --git a/source/libs/scalar/src/sclfunc.c b/source/libs/scalar/src/sclfunc.c
index 9aa67c441b..da4a39a551 100644
--- a/source/libs/scalar/src/sclfunc.c
+++ b/source/libs/scalar/src/sclfunc.c
@@ -2413,7 +2413,7 @@ int32_t toCharFunction(SScalarParam* pInput, int32_t inputNum, SScalarParam* pOu
 
     char *ts = colDataGetData(pInput[0].columnData, i);
     char *formatData = colDataGetData(pInput[1].columnData, pInput[1].numOfRows > 1 ? i : 0);
-    len = TMIN(TS_FORMAT_MAX_LEN - 1, varDataLen(formatData));
+    len = TMIN(TS_FORMAT_MAX_LEN - VARSTR_HEADER_SIZE, varDataLen(formatData));
     if (pInput[1].numOfRows > 1 || i == 0) {
       (void)strncpy(format, varDataVal(formatData), len);
       format[len] = '\0';