geom_hex/stat_hex binwidth argument no longer working

[coolbutuseless]

ggplot2.1.0

> ggplot(mtcars) + geom_hex(aes(mpg, mpg), bins=5)
# works fine

> ggplot(mtcars) + geom_hex(aes(mpg, mpg), binwidth=2)
Warning message:
Computation failed in `stat_binhex()`:
missing value where TRUE/FALSE needed 

May be related to: #1608