diff --git a/icefall/shared/make_kn_lm.py b/icefall/shared/make_kn_lm.py
index fe514e52b..a8b58dc30 100755
--- a/icefall/shared/make_kn_lm.py
+++ b/icefall/shared/make_kn_lm.py
@@ -243,7 +243,10 @@ class NgramCounts:
                         for u in a_counts_for_hist.word_to_count.keys():  # Should be careful here: what is Z1
                             sum_z1_f_z += _counts_for_hist.word_to_f[u]
 
-                        if 1.0 - sum_z1_f_z == 0:
+                        # I assume the following to be true:
+                        # assert sum_z1_f_z <= 1.0
+                        # assert sum_z1_f_a_z <= 1.0
+                        if sum_z1_f_z < 1:
                             counts_for_hist.word_to_bow[w] = (1.0 - sum_z1_f_a_z) / (1.0 - sum_z1_f_z)
                         else:
                             counts_for_hist.word_to_bow[w] = None