diff --git a/picard/metadata.py b/picard/metadata.py
index 773b7f8ae..f06a812ca 100644
--- a/picard/metadata.py
+++ b/picard/metadata.py
@@ -206,6 +206,8 @@ class Metadata(MutableMapping):
 
     @staticmethod
     def length_score(a, b):
+        if a is None or b is None:
+            return 0.0
         return (1.0 - min(abs(a - b),
                 LENGTH_SCORE_THRES_MS) / float(LENGTH_SCORE_THRES_MS))
 
diff --git a/test/test_metadata.py b/test/test_metadata.py
index 0bcf49016..0a74d4bb6 100644
--- a/test/test_metadata.py
+++ b/test/test_metadata.py
@@ -311,7 +311,10 @@ class CommonTests:
                        (20000, 20000, 1.0),
                        (20000, 30000, 0.666666666667),
                        (20000, 40000, 0.333333333333),
-                       (20000, 50000, 0.0)]
+                       (20000, 50000, 0.0),
+                       (20000, None, 0.0),
+                       (None, 2000, 0.0),
+                       (None, None, 0.0)]
             for (a, b, expected) in results:
                 actual = Metadata.length_score(a, b)
                 self.assertAlmostEqual(expected, actual,