fpp doesn't work when using Windows path separators

When trying to manually run fpp from a command line on a file in a subfolder, I noticed that it doesn't work right if using the '\' Windows path separator. So:

fpp src\test.F90

Gives the error: src\test.F90(3): #error: can't find include file: test.inc

The file in question is just

module test

#include "test.inc"

end module test

While

fpp src/test.F90

works fine. Bug or feature?