diff --git a/include/mio/detail/mmap.ipp b/include/mio/detail/mmap.ipp
index 69f070e..0b89178 100644
--- a/include/mio/detail/mmap.ipp
+++ b/include/mio/detail/mmap.ipp
@@ -124,7 +124,7 @@ file_handle_type open_file(const String& path, const access_mode mode,
     return handle;
 }
 
-inline int64_t query_file_size(file_handle_type handle, std::error_code& error)
+inline size_t query_file_size(file_handle_type handle, std::error_code& error)
 {
     error.clear();
 #ifdef _WIN32
@@ -318,7 +318,7 @@ void basic_mmap<AccessMode, ByteT>::map(const handle_type handle,
         return;
     }
 
-    const size_t file_size = detail::query_file_size(handle, error);
+    const auto file_size = detail::query_file_size(handle, error);
     if(error)
     {
         return;