Warning produced when using geom_blank() with another geom

Hi,

After updating ggplot2 I now get the following warning when I add "geom_blank" on other geoms, for example:

``` R
ggplot() + geom_bar(data=mtcars, aes(x=cyl, y=hp), stat="identity") + geom_blank()
```

will produce this warning:

``` R
Warning message:
In max(vapply(evaled, length, integer(1))) :
  no non-missing arguments to max; returning -Inf
```

Any ideas what this is referring to? 
