diff --git a/src/tsdb/src/tsdbRead.c b/src/tsdb/src/tsdbRead.c
index 3daefc1f5f0b74da98f926d650006f2bce46e23d..f88cd7190d729bc024f63e1585e50d6a20ceac9e 100644
--- a/src/tsdb/src/tsdbRead.c
+++ b/src/tsdb/src/tsdbRead.c
@@ -2311,7 +2311,11 @@ void filterPrepare(void* expr, void* param) {
   if (pInfo->optr == TSDB_RELATION_IN) {
     pInfo->q = (char*) pCond->arr;
   } else {
-    pInfo->q = calloc(1, pSchema->bytes + TSDB_NCHAR_SIZE);   // to make sure tonchar does not cause invalid write, since the '\0' needs at least sizeof(wchar_t) space.
+    uint32_t size = pCond->nLen * TSDB_NCHAR_SIZE;
+    if (size < (uint32_t)pSchema->bytes) {
+      size = pSchema->bytes;
+    }
+    pInfo->q = calloc(1, size + TSDB_NCHAR_SIZE);   // to make sure tonchar does not cause invalid write, since the '\0' needs at least sizeof(wchar_t) space.
     tVariantDump(pCond, pInfo->q, pSchema->type, true);
   }
 }