diff --git a/hexfont.lua b/hexfont.lua
index bffb778..45b613c 100755
--- a/hexfont.lua
+++ b/hexfont.lua
@@ -81,7 +81,7 @@ end
 --    hexfont.load_glyphs(io.lines("unifont.hex"))
 --    hexfont.load_glyphs(io.lines("unifont_upper.hex"))
 hexfont.load_glyphs = function(self, iterator)
-   assert( "function" == type(iterator) )
+   assert( "function" == type(iterator), "Are you using io.lines()?" )
    for line in iterator do
       assert("string" == type(line))
       local codepoint_hex, bitmap_hex = line:match(