diff --git a/src/md5.h b/src/md5.h
index a0a70f9..7b04d76 100644
--- a/src/md5.h
+++ b/src/md5.h
@@ -61,7 +61,7 @@ private:
         uint32_t B = value::kB;
         uint32_t C = value::kC;
         uint32_t D = value::kD;
-        size_t size = 0; // processed size in byte
+        uint64_t size = 0; // processed size in byte
     };
 
     md5_ctx ctx_;