diff --git a/core/testcasecontroller/metrics/metrics.py b/core/testcasecontroller/metrics/metrics.py
index bc9ed2938..a333fdf14 100644
--- a/core/testcasecontroller/metrics/metrics.py
+++ b/core/testcasecontroller/metrics/metrics.py
@@ -65,9 +65,7 @@ def compute(key, matrix):
             break
 
     if not flag:
-        bwt_score = np.nan
-        fwt_score = np.nan
-        return bwt_score, fwt_score
+        return None, np.nan, np.nan
 
     for i in range(length - 1):
         for j in range(length - 1):