Error from function

Debugger entered--Lisp error: (wrong-type-argument integerp nil)
  substring("abc def" nil 1)
  (let ((first-char (substring string nil 1)) (rest-str (substring string 1))) (concat (capitalize first-char) rest-str))
  (progn (let ((first-char (substring string nil 1)) (rest-str (substring string 1))) (concat (capitalize first-char) rest-str)))
  (if (and string (> (length string) 0)) (progn (let ((first-char (substring string nil 1)) (rest-str (substring string 1))) (concat (capitalize first-char) rest-str))))
  my/capitalize-first-char("abc def")
  eval((my/capitalize-first-char "abc def") nil)
  eval-expression((my/capitalize-first-char "abc def") nil)
  call-interactively(eval-expression nil nil)
  command-execute(eval-expression)