Fractional pen width should be allowed

[ccshan]

According to the documentation, the following code should succeed because the width of a pen just needs to be (and/c real? (<=/c 0 255)):

(require 2htdp/image)
(circle 50 "outline" (pen "red" 1.5 "solid" "round" "round"))

But I get pen: expects an integer between 0 and 255 as second argument, given 1.5

[rfindler]

Thanks!