diff --git a/gfx.h b/gfx.h
index 58cf6dea98..965fd6e044 100644
--- a/gfx.h
+++ b/gfx.h
@@ -72,7 +72,11 @@ void ToggleFullScreen(const bool full_screen);
 #define ASCII_LETTERSTART 32
 VARDEF int _stringwidth_base;
 VARDEF byte _stringwidth_table[0x2A0];
-static inline byte GetCharacterWidth(int key) { return _stringwidth_table[key - ASCII_LETTERSTART];}
+static inline byte GetCharacterWidth(uint key)
+{
+	assert(key >= ASCII_LETTERSTART && key - ASCII_LETTERSTART < lengthof(_stringwidth_table));
+	return _stringwidth_table[key - ASCII_LETTERSTART];
+}
 
 VARDEF DrawPixelInfo _screen;
 VARDEF DrawPixelInfo *_cur_dpi;