Problem with PG_GETARG_CSTRING

I'm probably just being a numpty, but I can't seem to get arguments with PG_GETARG_CSTRING.
I would much appreciate it if someone could point me in the right direction!

  The following code is simply test stuff and doesn't actually do anything other than raise notices

Thanks,
Brett

#include <strings.h>

#include "postgres.h"
#include "fmgr.h"
#include "executor/spi.h"

//PG_MODULE_MAGIC;

Datum bm_test(PG_FUNCTION_ARGS);

PG_FUNCTION_INFO_V1(bm_test);

Datum
bm_test(PG_FUNCTION_ARGS)
{
HeapTuple     tuple = NULL;
TupleDesc     tupdesc;
SPITupleTable *tuptable;
char          *szHostName = NULL;
int           i, ret;

//  palloc
  char *szPolicy = PG_GETARG_CSTRING(0);

elog(NOTICE, "szPolicy len [%d]'%s'\n", strlen(szPolicy), szPolicy);

  /*  Connect to the SPI Manager */
  if ((ret = SPI_connect()) < 0)
    /* internal error */
    elog(ERROR, "check_primary_key: SPI_connect returned %d", ret);

  /* Execute SELECT */
  if((ret = SPI_exec("SELECT sv_HostName FROM servers", 0)) != SPI_OK_SELECT) {
    /* CASE Error results ?, external function ? */
    elog(ERROR, "SELECT Failed, SP_exec returned %d.", ret);
  }

  while (SPI_tuptable->vals[i] != NULL)
  {
    tupdesc  = SPI_tuptable->tupdesc;
    tuptable = SPI_tuptable;
    tuple    = SPI_tuptable->vals[i];
    i++;

    szHostName = SPI_getvalue(tuple, tupdesc, 1);
    elog(NOTICE, "HostName %s", szHostName);
  }

  SPI_finish();
  PG_RETURN_NULL();
}

Compiled with:

gcc -Wall -Wmissing-prototypes -fpic -I/usr/include/pgsql/server/ -c bkptime.c
gcc -shared -o bkptime.so bkptime.o

Function created with:

create function bm_test(CSTRING) returns TEXT as '/home/brettm/BkpTime/bkptime.so' LANGUAGE 'c';

Executing the function returns the following:

ServerStats=# select * from bm_test('PRODAPP');
NOTICE:  szPolicy len [1]'
                          '

NOTICE:  HostName .......