Incompatible ranks?!

Here's a fun problem that I have encountered. It might be a bug, or it might not (I've run out of patience with google-ing it now I have my program working).

I have a two dimensional matrix. I sum it down one dimension and then find the position of the maximum entry.

subroutine ionfinder(subframe,x_dim,y_dim, irow, icol)

! define imputs, and function type itself
integer, intent(in) :: x_dim, y_dim
integer, intent(in) :: subframe(x_dim,y_dim)
integer, intent(out) :: irow, icol(4) !ion positions

! the code follows...*********************
irow = maxloc(sum(subframe,1))
write(*,*) 'ions in row (y): ', irow

end subroutine ionfinder

...but this causes my compiler (gfortran) to shriek:

irow = maxloc(sum(subframe,1))
1
Error: Incompatible ranks 0 and 1 in assignment at (1)

Huuuurrrmmm. Seems a bit silly, but the following kludge fixes things:

subroutine ionfinder(subframe,x_dim,y_dim, irow, icol)

! define imputs, and function type itself
integer, intent(in) :: x_dim, y_dim
integer, intent(in) :: subframe(x_dim,y_dim)
integer, intent(out) :: irow, icol(4) !ion positions

! define other datum within function
integer tmp(1)

! the code follows...*********************
tmp = maxloc(sum(subframe,1))
irow = tmp(1)
write(*,*) 'ions in row (y): ', irow

end subroutine ionfinder

Well well well. I should probably investigate the cause further but I'm not going to because I have physics to do, not Fortran to fix.