diff --git a/td/telegram/files/FileBitmask.cpp b/td/telegram/files/FileBitmask.cpp
index a0d9fdd1f..a6a0078ca 100644
--- a/td/telegram/files/FileBitmask.cpp
+++ b/td/telegram/files/FileBitmask.cpp
@@ -52,6 +52,10 @@ int64 Bitmask::get_ready_prefix_size(int64 offset, int64 part_size, int64 file_s
   if (offset < 0) {
     return 0;
   }
+  if (part_size == 0) {
+    return 0;
+  }
+  CHECK(part_size > 0);
   auto offset_part = offset / part_size;
   auto ones = get_ready_parts(offset_part);
   if (ones == 0) {