include works with if but not else

I doing a Ruby botcamp. I'm supposed to write code that replaces all user input of 's' with 'th' so it reads like Daffy Duck is speaking. If I enter an s it will be replaced with th. That works! But If I don't enter an 's' it's supposed to print that none were included in my elsif statemnt. Instead I'm getting the error 'undefined method `include?' for nil:NilClass'. Other than that error, the interpretor is telling me the code is good.

print "Input a string: "
user_input=gets.chomp.downcase!
if user_input.include?"s"
user_input.gsub!(/s/, "th")
puts "Your string is #{user_input}!"
elsif
puts "There are no s's in your string!"
end

Any ideas on what I need to change?