diff --git a/include/doorkeeper/implem/tileiterator.inl b/include/doorkeeper/implem/tileiterator.inl
index 92c5e67..c242040 100644
--- a/include/doorkeeper/implem/tileiterator.inl
+++ b/include/doorkeeper/implem/tileiterator.inl
@@ -44,7 +44,8 @@ namespace dk {
 	T& TileIterator<T, D, T1>::dereference() const {
 		const auto index = to_index(m_tile_range);
 		DK_ASSERT(m_data);
-		DK_ASSERT(index < m_data->size());
+		DK_ASSERT(index >= 0);
+		DK_ASSERT(static_cast<typename std::make_unsigned<decltype(index)>::type>(index) < m_data->size());
 		return (*m_data)[index];
 	}