diff --git a/regression-test/suites/compaction/test_vertical_compaction_agg_keys.groovy b/regression-test/suites/compaction/test_vertical_compaction_agg_keys.groovy
index 39bc34333f..2c7abd178e 100644
--- a/regression-test/suites/compaction/test_vertical_compaction_agg_keys.groovy
+++ b/regression-test/suites/compaction/test_vertical_compaction_agg_keys.groovy
@@ -118,7 +118,7 @@ suite("test_vertical_compaction_agg_keys") {
         // trigger compactions for all tablets in ${tableName}
         for (String[] tablet in tablets) {
             String tablet_id = tablet[0]
-            backend_id = tablet[2]
+            String backend_id = tablet[2]
             (code, out, err) = be_run_cumulative_compaction(backendId_to_backendIP.get(backend_id), backendId_to_backendHttpPort.get(backend_id), tablet_id)
             logger.info("Run compaction: code=" + code + ", out=" + out + ", err=" + err)
             assertEquals(code, 0)
@@ -138,6 +138,7 @@ suite("test_vertical_compaction_agg_keys") {
             do {
                 Thread.sleep(1000)
                 String tablet_id = tablet[0]
+                String backend_id = tablet[2]
                 (code, out, err) = be_get_compaction_status(backendId_to_backendIP.get(backend_id), backendId_to_backendHttpPort.get(backend_id), tablet_id)
                 logger.info("Get compaction status: code=" + code + ", out=" + out + ", err=" + err)
                 assertEquals(code, 0)