Using (sqrt x) in Lisp code

Using (sqrt x) in Lisp code

I am having some issue while writing symbolic differentiation in lisp. I
am trying write derivative of sqrt (x) but when i use this variable inside
code, it give me that x is not defined.
;----------------------------------------
; deriv sqrt
;----------------------------------------
(defun derivsqrt(expr var)
(smult
(smult
(sdiv 1 2)
(sqrt (second expr)) ; This line gives me error
)
(deriv (second expr) var)
)
)
Can somebody help?