diff --git a/src/tsdb/src/tsdbRead.c b/src/tsdb/src/tsdbRead.c
index 71a9009c4581dae86861a67de850e275cae352f7..e3ad254bbb1a4ce0a3c504e9a799a109b7e1159a 100644
--- a/src/tsdb/src/tsdbRead.c
+++ b/src/tsdb/src/tsdbRead.c
@@ -3474,19 +3474,19 @@ void filterPrepare(void* expr, void* param) {
 
   if (pInfo->optr == TSDB_RELATION_IN) {
      int dummy = -1;
-     SHashObj *pObj = NULL; 
+     SHashObj *pObj = NULL;
      if (pInfo->sch.colId == TSDB_TBNAME_COLUMN_INDEX) {
         pObj = taosHashInit(256, taosGetDefaultHashFunction(pInfo->sch.type), true, false);
         SArray *arr = (SArray *)(pCond->arr);
         for (size_t i = 0; i < taosArrayGetSize(arr); i++) {
           char* p = taosArrayGetP(arr, i);
-          strtolower(varDataVal(p), varDataVal(p));
-          taosHashPut(pObj, varDataVal(p),varDataLen(p), &dummy, sizeof(dummy));
+          strntolower_s(varDataVal(p), varDataVal(p), varDataLen(p));
+          taosHashPut(pObj, varDataVal(p), varDataLen(p), &dummy, sizeof(dummy));
         }
      } else {
        buildFilterSetFromBinary((void **)&pObj, pCond->pz, pCond->nLen);
      }
-     pInfo->q = (char *)pObj;  
+     pInfo->q = (char *)pObj;
   } else if (pCond != NULL) {
     uint32_t size = pCond->nLen * TSDB_NCHAR_SIZE;
     if (size < (uint32_t)pSchema->bytes) {
diff --git a/src/util/inc/tutil.h b/src/util/inc/tutil.h
index 7c8fd2ed8353a086ec9f145aaaa30f4db57113cb..6bcfb5de295c5719032b81c23d16ec2b1476349e 100644
--- a/src/util/inc/tutil.h
+++ b/src/util/inc/tutil.h
@@ -32,6 +32,7 @@ char *  strnchr(char *haystack, char needle, int32_t len, bool skipquote);
 char ** strsplit(char *src, const char *delim, int32_t *num);
 char *  strtolower(char *dst, const char *src);
 char *  strntolower(char *dst, const char *src, int32_t n);
+char *  strntolower_s(char *dst, const char *src, int32_t n);
 int64_t strnatoi(char *num, int32_t len);
 char *  strbetween(char *string, char *begin, char *end);
 char *  paGetToken(char *src, char **token, int32_t *tokenLen);
diff --git a/src/util/src/tutil.c b/src/util/src/tutil.c
index 02ea7091a65cc0decd3030b8805de9a7a1fed419..5f8c92898fc5f0abc4c733c0558befd68ac3cac7 100644
--- a/src/util/src/tutil.c
+++ b/src/util/src/tutil.c
@@ -202,7 +202,7 @@ char* strntolower(char *dst, const char *src, int32_t n) {
   if (n == 0) {
     *p = 0;
     return dst;
-  } 
+  }
   for (c = *src++; n-- > 0; c = *src++) {
     if (esc) {
       esc = 0;
@@ -224,6 +224,26 @@ char* strntolower(char *dst, const char *src, int32_t n) {
   return dst;
 }
 
+char* strntolower_s(char *dst, const char *src, int32_t n) {
+  char *p = dst, c;
+
+  assert(dst != NULL);
+  if (n == 0) {
+    return NULL;
+  }
+
+  while (n-- > 0) {
+    c = *src;
+    if (c >= 'A' && c <= 'Z') {
+      c -= 'A' - 'a';
+    }
+    *p++ = c;
+    src++;
+  }
+
+  return dst;
+}
+
 char *paGetToken(char *string, char **token, int32_t *tokenLen) {
   char quote = 0;