fix r62970 as r62990

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@63007 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/tmpdir.rb b/lib/tmpdir.rb
index 7c2651efbb..7c0ce3a7f0 100644
--- a/lib/tmpdir.rb
+++ b/lib/tmpdir.rb
@@ -114,9 +114,9 @@ class Dir
       end
       n = nil
       prefix, suffix = basename
-      prefix = prefix.delete("#{File::SEPARATOR}#{File::ALT_SEPARATOR}")
       prefix = (String.try_convert(prefix) or
                 raise ArgumentError, "unexpected prefix: #{prefix.inspect}")
+      prefix = prefix.delete("#{File::SEPARATOR}#{File::ALT_SEPARATOR}")
       suffix &&= (String.try_convert(suffix) or
                   raise ArgumentError, "unexpected suffix: #{suffix.inspect}")
       suffix &&= suffix.delete("#{File::SEPARATOR}#{File::ALT_SEPARATOR}")