diff --git a/src/client/src/tscSQLParser.c b/src/client/src/tscSQLParser.c
index 819f1af4f0396e1e65952418636bb994efc58cb7..27b839169cef6c7347c4877b7a3f7ad15e25a2bf 100644
--- a/src/client/src/tscSQLParser.c
+++ b/src/client/src/tscSQLParser.c
@@ -6078,10 +6078,12 @@ int32_t setAlterTableInfo(SSqlObj* pSql, struct SSqlInfo* pInfo) {
 
     SSchema* pSchema = tscGetTableTagSchema(pTableMetaInfo->pTableMeta);
     int16_t numOfTags = tscGetNumOfTags(pTableMetaInfo->pTableMeta);
-    int16_t i;
+    int32_t numOfCols = tscGetNumOfColumns(pTableMetaInfo->pTableMeta);
+    int32_t tagIndex = columnIndex.columnIndex - numOfCols;
+    assert(tagIndex>=0);
     uint32_t nLen = 0;
-    for (i = 0; i < numOfTags; ++i) {
-      nLen += (i != columnIndex.columnIndex) ? pSchema[i].bytes : pItem->bytes;
+    for (int i = 0; i < numOfTags; ++i) {
+      nLen += (i != tagIndex) ? pSchema[i].bytes : pItem->bytes;
     }
     if (nLen >= TSDB_MAX_TAGS_LEN) {
       return invalidOperationMsg(pMsg, msg24);
diff --git a/tests/script/general/parser/alter_stable.sim b/tests/script/general/parser/alter_stable.sim
index afdd7d3edf1ba4f2159569f497bf495cce8ec650..1406af608779eef2048de24b6e4aef6b99b54d7d 100644
--- a/tests/script/general/parser/alter_stable.sim
+++ b/tests/script/general/parser/alter_stable.sim
@@ -35,7 +35,10 @@ sql alter table tb1 set tag name = ""
 sql alter table tb1 set tag name = "shenzhen"
 sql alter table tb1 set tag len = 379
 
+# case TD-5594
+sql create stable st5520(ts timestamp, f int) tags(t0 bool, t1 nchar(4093), t2 nchar(1))
+sql_error alter stable st5520 modify tag t2 nchar(2);
 # test end   
 sql drop database $db
 
-system sh/exec.sh -n dnode1 -s stop -x SIGINT
\ No newline at end of file
+system sh/exec.sh -n dnode1 -s stop -x SIGINT