diff --git a/lib/mail_catcher.rb b/lib/mail_catcher.rb
index f47115c..cb142be 100644
--- a/lib/mail_catcher.rb
+++ b/lib/mail_catcher.rb
@@ -9,7 +9,7 @@ require 'mail_catcher/version'
 
 module MailCatcher extend self
   def which command
-    Open3.popen3 'which', 'command' do |stdin, stdout, stderr|
+    not windows? and Open3.popen3 'which', 'command' do |stdin, stdout, stderr|
       return stdout.read.chomp.presence
     end
   end