diff --git a/src/query/src/qExecutor.c b/src/query/src/qExecutor.c
index e3a0f159490907c9ea9ec15a396b2f0257a9a524..2dff94bee0e9381b7d4feb51f7df92b0dcd0ee9c 100644
--- a/src/query/src/qExecutor.c
+++ b/src/query/src/qExecutor.c
@@ -2206,8 +2206,18 @@ static void doSetTagValueInParam(void *tsdb, STableId* pTableId, int32_t tagColI
     }
     
     if (type == TSDB_DATA_TYPE_BINARY || type == TSDB_DATA_TYPE_NCHAR) {
+      if (isNull(varDataVal(val), type)) {
+        tag->nType = TSDB_DATA_TYPE_NULL;
+        return;
+      }
+
       tVariantCreateFromBinary(tag, varDataVal(val), varDataLen(val), type);
     } else {
+      if (isNull(val, type)) {
+        tag->nType = TSDB_DATA_TYPE_NULL;
+        return;
+      }
+
       tVariantCreateFromBinary(tag, val, bytes, type);
     }
   }
@@ -4285,12 +4295,12 @@ static void sequentialTableProcess(SQInfo *pQInfo) {
       assert(taosArrayGetSize(s) >= 1);
       
       setTagVal(pRuntimeEnv, (STableId*) taosArrayGet(s, 0), pQInfo->tsdb);
-        
       if (isFirstLastRowQuery(pQuery)) {
         assert(taosArrayGetSize(s) == 1);
       }
+
       taosArrayDestroy(s);
-      
+
       // here we simply set the first table as current table
       pQuery->current = ((SGroupItem*) taosArrayGet(group, 0))->info;
       scanOneTableDataBlocks(pRuntimeEnv, pQuery->current->lastKey);
diff --git a/src/tsdb/src/tsdbRead.c b/src/tsdb/src/tsdbRead.c
index 93f5ca93557111afd1d739540b162a00248d6880..732e8e900812acff6bc18ea7cd4277b74d1b917f 100644
--- a/src/tsdb/src/tsdbRead.c
+++ b/src/tsdb/src/tsdbRead.c
@@ -1930,8 +1930,7 @@ int32_t tableGroupComparFn(const void *p1, const void *p2, const void *param) {
     SColIndex* pColIndex = &pTableGroupSupp->pCols[i];
     int32_t colIndex = pColIndex->colIndex;
     
-    assert((colIndex >= 0 && colIndex < schemaNCols(pTableGroupSupp->pTagSchema)) ||
-           (colIndex == TSDB_TBNAME_COLUMN_INDEX));
+    assert(colIndex >= TSDB_TBNAME_COLUMN_INDEX);
     
     char *  f1 = NULL;
     char *  f2 = NULL;
@@ -1950,7 +1949,20 @@ int32_t tableGroupComparFn(const void *p1, const void *p2, const void *param) {
       f1 = tdGetKVRowValOfCol(pTable1->tagVal, pCol->colId);
       f2 = tdGetKVRowValOfCol(pTable2->tagVal, pCol->colId);
     }
-    
+
+    // this tags value may be NULL
+    if (f1 == NULL && f2 == NULL) {
+      continue;
+    }
+
+    if (f1 == NULL) {
+      return -1;
+    }
+
+    if (f2 == NULL) {
+      return 1;
+    }
+
     int32_t ret = doCompare(f1, f2, type, bytes);
     if (ret == 0) {
       continue;
diff --git a/tests/script/general/parser/set_tag_vals.sim b/tests/script/general/parser/set_tag_vals.sim
index a684fd5406950ede213ad065a45a7aa79d355818..9d7ac9678daefde512ac9d126742b43aa3508017 100644
--- a/tests/script/general/parser/set_tag_vals.sim
+++ b/tests/script/general/parser/set_tag_vals.sim
@@ -9,7 +9,7 @@ sql connect
 $dbPrefix = db
 $tbPrefix = tb
 $stbPrefix = stb
-$tbNum = 1000
+$tbNum = 100
 $rowNum = 100
 $totalNum = $tbNum * $rowNum
 $ts0 = 1537146000000
@@ -26,7 +26,7 @@ $stb = $stbPrefix . $i
 sql drop database $db -x step1
 step1:
 sql create database $db 
-print ====== create tables
+print ====== create $tbNum tables
 sql use $db
 sql create table $stb (ts timestamp, c1 int, c2 bigint, c3 float, c4 double, c5 smallint, c6 tinyint, c7 bool, c8 binary(10), c9 nchar(10)) tags(t1 binary(9), t2 binary(8))