Debugger entered--Lisp error: (wrong-type-argument stringp nil)

[damon-kwok]

OS: ubuntu 16.04.3
Emacs: 27.0.50
Error:

(require 'helm-ros)

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
string-match(":" nil 0)
split-string(nil ":")
byte-code("\300\301\302\303\304\305\306!\307""\310#\207" [mapconcat identity cl-remove-if-not file-exists-p split-string getenv "ROS_PACKAGE_PATH" ":" " "] 7)
(defvar helm-ros--package-path (byte-code "\300\301\302\303\304\305\306!\307""\310#\207" [mapconcat identity cl-remove-if-not file-exists-p split-string getenv "ROS_PACKAGE_PATH" ":" " "] 7))
require(helm-ros)
(progn (require 'helm-ros))
eval((progn (require 'helm-ros)) t)
elisp--eval-last-sexp(nil)
eval-last-sexp(nil)
funcall-interactively(eval-last-sexp nil)
call-interactively(eval-last-sexp nil nil)
command-execute(eval-last-sexp)