C51: Null function pointer

I am trying to convert an unmaintainable user interface
driver over to something that uses data structures to define
the menu organization (This example is stripped down to the
bare minimum from my actual app).

Picture an array of screens, selectable by the user's
activation of some arrow keys:

         Col 0                  Col 1
           +-----------+
    Row 0  | Screen 1a |
           +-----------+
                /\ 
                ||
                \/
           +-----------+       +-----------+
    Row 1  | Screen 2a | <===> | Screen 2b |
           +-----------+       +-----------+

I define a function to output each screen (in the code
below, they also change the row & column indices):

  void f1a(void) {

    outputScreen1a();
    if(r > 0)
      r++;
    return;
  }

  /* ============= */

  void f2a(void) {

    outputScreen2a();
    if(c < 1)
      c++;
    return;
  }

  /* ============= */

  void f2b(void) {

    outputScreen2b();
    r = c = 0;
    return;
  }

These are called by the main() function, using a function
pointer table:

  static code const void (* code menuFunc[2][2])(void) = {
    { f1a,    NULL },
    { f2a,    f2b  }
  };

  unsigned char r, c;

  void main(void) {
    unsigned char i;

    SFRPAGE = 0;   /* Go to default page     */
    WDTCN = 0xDE;  /* Disable watchdog timer */
    WDTCN = 0xAD;
    IE   = 0;      /* Disable all interrupts */
    i = 0;
    r = c = 0;

    while (1) {

      if(i < 70) {
        i++;
      } else {
        i = 0;
                  (*menuFunc[r][c])();
      }
    }  /* END while (TRUE) */

  }  /* END main() */

As presented above, the source file will compile without
error. But my application is quite a bit more complex than
what I present here, the table is assymetrical (marked by
the NULL pointer) and I want to guantee I do not
inadvertently dereference it, so I add a qualification to
the main control loop:

      while (1) {

        if(i < 70) {
          i++;
        } else {
          i = 0;
          if((menuFunc[r][c])() != NULL)
                    (*menuFunc[r][c])();
        }
      }  /* END while (TRUE) */

C51 does not like this. It flags the statement where I check
for NULL:

?????.C:(56): error C193: '!=' : illegal conversion from/to 'void'

NULL is defined in stdio.h:

 #define NULL ((void *) 0)

K&R doesn't state it explicitly, but seems to assume that
void is an object type, that (void *) can only point to an
object, and never uses NULL as a pointer to a function.

So the question is, if I can't use NULL to mark an
uncallable function in the table, is there a null function
pointer I could use instead?

============================================================
Gary Lynch           |   To send mail, no$pam in domain name
lynchg@no$pam.com    |   must be changed to stacoenergy.
============================================================