Fortran C interoperability: Returning string from Fortran to C

Hi, I'm building a C wrapper for my Fortran simulator to be used in Python. I made this function that takes the number of an element and returns its name:

type(c_ptr) function get_elem_name(i) bind(C, name="get_elem_name")
#IFDEF DLL
!DEC$ ATTRIBUTES DLLEXPORT :: get_elem_name
#ENDIF
  use ELEMENTS, only: elemname, nbelem
  implicit none
  integer(c_int), INTENT(IN), value :: i
  character(len=21,kind=c_char) :: returnval
  returnval=''//C_NULL_CHAR
  if(i<=nbelem .and. i>0)returnval=trim(elemname(i))//C_NULL_CHAR
  get_elem_name = C_LOC(returnval)
end function get_elem_name

It is called from Python as:

import ctypes
mylib = ctypes.CDLL("mylib.dll")
mylib.init() # initialize simulator
mylib.get_elem_name(1) # get the name of the first element

It works perfectly for the first 200 calls or something. After that, it randomly returns an empty string (''). I cannot get my head around the problem. Could it be that retval is automatically allocated inside Fortran and then the pointer passed to Python? Could that create an issue?

Thanks for any help.