diff --git a/td/telegram/files/FileBitmask.cpp b/td/telegram/files/FileBitmask.cpp
index 0c6af8423..1ff6cb28f 100644
--- a/td/telegram/files/FileBitmask.cpp
+++ b/td/telegram/files/FileBitmask.cpp
@@ -20,9 +20,12 @@ Bitmask::Bitmask(Ones, int64 count) : data_(narrow_cast<size_t>((count + 7) / 8)
   }
 }
 
-std::string Bitmask::encode() const {
+std::string Bitmask::encode(int32 prefix_count) const {
   // remove zeroes in the end to make encoding deterministic
   td::Slice data(data_);
+  if (prefix_count != -1) {
+    data.truncate(prefix_count);
+  }
   while (!data.empty() && data.back() == '\0') {
     data.remove_suffix(1);
   }
diff --git a/td/telegram/files/FileBitmask.h b/td/telegram/files/FileBitmask.h
index 0fd6427a4..ef17c0a4d 100644
--- a/td/telegram/files/FileBitmask.h
+++ b/td/telegram/files/FileBitmask.h
@@ -19,7 +19,7 @@ class Bitmask {
   Bitmask() = default;
   Bitmask(Decode, Slice data);
   Bitmask(Ones, int64 count);
-  std::string encode() const;
+  std::string encode(int32 prefix_count = -1) const;
   int64 get_ready_prefix_size(int64 offset, int64 part_size, int64 file_size) const;
   int64 get_total_size(int64 part_size, int64 file_size) const;
   bool get(int64 offset_part) const;
diff --git a/td/telegram/files/PartsManager.cpp b/td/telegram/files/PartsManager.cpp
index 386d6b864..5799c8fea 100644
--- a/td/telegram/files/PartsManager.cpp
+++ b/td/telegram/files/PartsManager.cpp
@@ -170,7 +170,11 @@ int32 PartsManager::get_ready_prefix_count() {
   return res;
 }
 string PartsManager::get_bitmask() const {
-  return bitmask_.encode();
+  int32 prefix_count = -1;
+  if (need_check_) {
+    prefix_count = narrow_cast<int32>(checked_prefix_size_ / part_size_);
+  }
+  return bitmask_.encode(prefix_count);
 }
 
 Result<Part> PartsManager::start_part() {