diff --git a/icefall/utils.py b/icefall/utils.py
index 0c509fc17..1dbb954de 100644
--- a/icefall/utils.py
+++ b/icefall/utils.py
@@ -1159,7 +1159,8 @@ class MetricsTracker(collections.defaultdict):
         for k, v in self.items():
             ans[k] = v
         for k, v in other.items():
-            ans[k] = ans[k] + v
+            if v - v == 0:
+                ans[k] = ans[k] + v
         return ans
 
     def __mul__(self, alpha: float) -> "MetricsTracker":