diff --git a/include/flatbuffers/allocator.h b/include/flatbuffers/allocator.h
index d451818568..e1c8fa7963 100644
--- a/include/flatbuffers/allocator.h
+++ b/include/flatbuffers/allocator.h
@@ -57,8 +57,12 @@ class Allocator {
   void memcpy_downward(uint8_t* old_p, size_t old_size, uint8_t* new_p,
                        size_t new_size, size_t in_use_back,
                        size_t in_use_front) {
+    FLATBUFFERS_ASSERT(in_use_back <= old_size);
+    FLATBUFFERS_ASSERT(in_use_back <= new_size);
     memcpy(new_p + new_size - in_use_back, old_p + old_size - in_use_back,
            in_use_back);
+    FLATBUFFERS_ASSERT(in_use_front <= old_size);
+    FLATBUFFERS_ASSERT(in_use_front <= new_size);
     memcpy(new_p, old_p, in_use_front);
   }
 };