diff --git a/source/paint/graphics.cpp b/source/paint/graphics.cpp
index 684a8ca2..aa606f4e 100644
--- a/source/paint/graphics.cpp
+++ b/source/paint/graphics.cpp
@@ -413,7 +413,7 @@ namespace paint
 
 		nana::size	graphics::text_extent_size(const std::wstring& text)	const
 		{
-			return text_extent_size(text.c_str(), text.length());
+			return text_extent_size(text.c_str(), static_cast<unsigned>(text.length()));
 		}
 
 		nana::size	graphics::text_extent_size(const wchar_t* str, std::size_t len)	const