diff --git a/common/ngram-map.cpp b/common/ngram-map.cpp
index b41f6bd8ab..8e3978f7ed 100644
--- a/common/ngram-map.cpp
+++ b/common/ngram-map.cpp
@@ -208,7 +208,7 @@ void common_ngram_map_begin(
                 count_keys, count_keys_del, count_values_del, count_map_entries_upd);
     }
 
-    map.idx_last_check = (map.size_last_begin > 0) ? map.size_last_begin - 1 : 0;
+    map.idx_last_check = size_begin;
     map.size_last_begin = size_begin;
 }