diff --git a/developer/bin/cask_pr_manual_check b/developer/bin/cask_pr_manual_check
index ae4650786d..248a08af15 100755
--- a/developer/bin/cask_pr_manual_check
+++ b/developer/bin/cask_pr_manual_check
@@ -6,7 +6,7 @@ require 'tmpdir'
 
 pr_url = ARGV[0]
 
-abort 'You need to give the script exactly one argument, the URL to a pull request from an official Homebrew Cask tap' if ARGV[0].empty?
+abort 'You need to give the script exactly one argument, the URL to a pull request from an official Homebrew Cask tap' if pr_url.nil?
 abort 'URL is not from an official Homebrew Cask tap' if pr_url !~ %r{^https://github.com/Homebrew/homebrew-cask.*}
 
 pr_api = pr_url.sub(%r{^https://github.com/([^/]+)/([^/]+)/pull/([^/]+).*}, 'https://api.github.com/repos/\1/\2/pulls/\3/files')