coord_trans with continuous_scale and minor_breaks = NULL causes error

[dbeltoft]

Hi

Noob here, apologize in advance if this is already known, or if I misunderstood a feature as a bug.

When trying to suppress minor breaks on my axis, while applying a coordinate transformation with coord_trans, I get an error. Here is a minimal example to reproduce the error:

library(ggplot2)
data <- data.frame(x = c(1,2,3), y = c(5,2,4))
ggplot(data, aes(x,y)) + 
  geom_point() +
  scale_x_continuous(minor_breaks = NULL) +
  coord_trans(xtrans = "log")                                  # This line causes the error
# Error in log(x, base) : non-numeric argument to mathematical function

Going through the traceback, it seems that transform_value does not check for NULL arguments, which makes it fail when trying to rescale the minor breaks.