diff --git a/internal/proxy/task_index.go b/internal/proxy/task_index.go
index f02bedc545c30..41f34836f8337 100644
--- a/internal/proxy/task_index.go
+++ b/internal/proxy/task_index.go
@@ -154,8 +154,8 @@ func (cit *createIndexTask) parseFunctionParamsToIndex(indexParamsMap map[string
 		}
 
 		if metricType, ok := indexParamsMap["metric_type"]; !ok {
-			indexParamsMap["metric_type"] = "BM25"
-		} else if metricType != "BM25" {
+			indexParamsMap["metric_type"] = metric.BM25
+		} else if metricType != metric.BM25 {
 			return fmt.Errorf("index metric type of BM25 function output field must be BM25, got %s", metricType)
 		}
 
diff --git a/internal/querynodev2/delegator/delegator.go b/internal/querynodev2/delegator/delegator.go
index 8fd8447914150..06f98608ecbe3 100644
--- a/internal/querynodev2/delegator/delegator.go
+++ b/internal/querynodev2/delegator/delegator.go
@@ -263,8 +263,13 @@ func (sd *shardDelegator) search(ctx context.Context, req *querypb.SearchRequest
 		}()
 	}
 
-	// build idf for bm25 search
-	if req.GetReq().GetMetricType() == metric.BM25 || (req.GetReq().GetMetricType() == metric.EMPTY && sd.isBM25Field[req.GetReq().GetFieldId()]) {
+	searchAgainstBM25Field := sd.isBM25Field[req.GetReq().GetFieldId()]
+
+	if searchAgainstBM25Field {
+		if req.GetReq().GetMetricType() != metric.BM25 && req.GetReq().GetMetricType() != metric.EMPTY {
+			return nil, merr.WrapErrParameterInvalid("BM25", req.GetReq().GetMetricType(), "must use BM25 metric type when searching against BM25 Function output field")
+		}
+		// build idf for bm25 search
 		avgdl, err := sd.buildBM25IDF(req.GetReq())
 		if err != nil {
 			return nil, err
diff --git a/internal/querynodev2/delegator/delegator_test.go b/internal/querynodev2/delegator/delegator_test.go
index cb541aa6e98c1..1bd61173c2aee 100644
--- a/internal/querynodev2/delegator/delegator_test.go
+++ b/internal/querynodev2/delegator/delegator_test.go
@@ -1267,3 +1267,23 @@ func TestDelegatorTSafeListenerClosed(t *testing.T) {
 	assert.Equal(t, sd.Serviceable(), false)
 	assert.Equal(t, sd.Stopped(), true)
 }
+
+func TestDelegatorSearchBM25InvalidMetricType(t *testing.T) {
+	paramtable.Init()
+	searchReq := &querypb.SearchRequest{
+		Req: &internalpb.SearchRequest{
+			Base: commonpbutil.NewMsgBase(),
+		},
+	}
+
+	searchReq.Req.FieldId = 101
+	searchReq.Req.MetricType = metric.IP
+
+	sd := &shardDelegator{
+		isBM25Field: map[int64]bool{101: true},
+	}
+
+	_, err := sd.search(context.Background(), searchReq, []SnapshotItem{}, []SegmentEntry{})
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "must use BM25 metric type when searching against BM25 Function output field")
+}