-working-dir flag ignored for C files

It seems that the -working-dir flag does not work correctly when C files are involved (or presumably other non-Haskell source files).

Example:

-- dir/Main.hs
{-# LANGUAGE ForeignFunctionInterface #-}
module Main where
foreign import ccall "hello"
  hello :: IO ()
main :: IO ()
main = hello

// dir/Aux.c
#include <stdio.h>
void hello() {
   printf("hi");
}

> ghc -working-dir dir Main.hs Aux.c
<command line>: does not exist: Aux.c
> cd dir && ghc Main.hs Aux.c
... works fine