visual lisp accepting default values?

This may seem basic, but I am having a problem getting a simple lisp routine to accept a default value.

when I enter the following code:

(setq dst1 (getdist "\nEnter Post to Post spacing : "))

and accept the default, I get the response:

; error: bad argument type: numberp: nil

What am I doing wrong? I know it's got to be simple, but I can't seem to find any reference to accepting a default value anywhere.

By the way, I am new at this. Did you notice?

Thanks,

Joe B.

(setq dst1 "5'4.5")

After the defining of the function

you could try this, input needs to be in inches

(if (null dst)
(setq dst 64.5)
);if
(setq dst1 (getdist (strcat "\nEnter Post to Post spacing : <" (rtos dst 3 1) ">: "))
);setq
(if (not dst1)
(setq dst1 dst)
(setq dst dst1)
);if

Here is my version:

  (or dst (setq dst 64.5)) ; set default
 (and (setq ans (getdist (strcat "\nEnter Post to Post spacing : <" (rtos dst 3 1) ">: ")))
      (setq dst ans)
 )

Thank guys... I keep thinking life is simple.