LISP, Bad Argument Type

I have problem to lsp programme run the programe autocad give ; error: bad argument type: numberp: nil error please help me

here is my programme

(DEFUN C:nallano()

(COMMAND "LAYER" "m" "nalano""c" "w""" "m" "cbarea""c" "r" """m" "boundary""c" "y" """")

(SETQ NANO(GETSTRING"\nENTER NALLA NUMBER :"))

(setq cbno(getreal"enter catch basin no :"))

(while (SETQ PT (GETPOINT"SELECT THE POINT FOR CALCULATING AREA OF BOUNDARY :"))

(COMMAND "LAYER" "S" "boundary" "")

(command"boundary" PT "")

(setq ENTl (entget (entlast)))

(command "area" "Object" "last")

(setq plotarea (getvar "area" ))

; (princ plotarea)(getreal"....")

; (command"erase" "last" "" )

;(COMMAND"DDATTE" PAUSE)

(WRITETEXT plotarea pt)

(setq chno (+ 1 chno))

)

)

(defun writetext(plotarea pt)

(SETQ MAXLV(GETSTRING"\ENTER THE Maximum High Level :"))

(SETQ NALVL(GETSTRING"\ENTER THE Nala Invert Level :"))

(setq orgpt pt)

(setq wrtpt(list (car orgpt)(+(cadr orgpt)2)))

(setq wrtpt1(list (car orgpt)(+(cadr orgpt)4)))

(command"insert" "nalano" pt "" "" "0" SHNO surno hno (rtos chno 2 0) (rtos plotarea 2 3) )

;(COMMAND "LAYER" "S" "chaltano" "")

;(command"text" "Ml" ORGPT 1 0 (rtos chno 1 0))

; (COMMAND "LAYER" "S" "hissa-number" "")

; (command"text" "MC" WRTPT 1 0 hno)

; (COMMAND "LAYER" "S" "charea" "")

; (command"text" "MC" WRTPT1 2 0 (rtos plotarea 2 3))

)

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You should check this line - the value for first 4 variables (in red) that you attempt to pass as attributes are evaluated to nil:

(command "insert" "nalano" pt "" "" "0" [color=red]SHNO surno hno[/color] (rtos [color=red]chno[/color] 2 0) (rtos plotarea 2 3))

Also this line would generate an error since the variable isn't initialized:

(setq chno (+ 1 chno))