Transferring characters / strings from R to Fortran

I have a Fortran routine that selects a function based on a string value and then executes that function.

!! file:select.f90
module funcs
    contains
        subroutine add(x, y, xy)
            real :: x, y, xy
            xy = x + y
            return
        end subroutine

        subroutine diff(x, y, xy)
            real :: x, y, xy
            xy = x - y
            return
        end subroutine
end module

subroutine simple(modname)
    use funcs

    procedure(), pointer :: model => null()

    character(10) :: modname
    real :: x, y, xy

    print *, "-",modname,"-"
    select case (modname)
        case("add")
            model => add
        case("diff")
            model => diff
        case default
            print *, "No model with that name!"
            stop
    end select

    x = 4
    y = 3
    call model(x, y, xy)
    print *, xy

end subroutine

      

        
        
        
      

    

I would like to call this subroutine from an R script.

# file:select.R
dyn.load("select.so")
.Fortran("simple", "add")
.Fortran("simple", "diff")

      

        
        
        
      

    

As a standalone Fortran program

that takes a command line argument, this works great. He is even insensitive to the spaces before or after modname

. However, when I try to pass a character as an argument from R, it retypes the character correctly (no extra whitespace), but then doesn't recognize it as case

and skips over it default

. What's going on here? Is there some issue with the R character encoding that makes them incompatible with Fortran?