NoMethodError: undefined method "delete_prefix" String class (Ruby 2.4.1)

Question

I am using the latest Ruby (version 2.4.1)

I see a method there String#delete_prefix, as defined in https://ruby-doc.org/core-2.4.1/String.html#method-i-delete_prefix

But in irbthis method for some reason does not exist:

2.4.1 :030 > c="_abc_xyz"
=> "_abc_xyz"
2.4.1 :031 > c.delete_prefix("_")
NoMethodError: undefined method `delete_prefix' for "_abc_xyz":String
from (irb):31
from /Users/jonsmith/.rvm/rubies/ruby-2.4.1/bin/irb:11:in `<main>'
2.4.1 :032 >

What's going on here?!

(The above can be achieved with help c.gsub(/^_/, ''), but I wonder why delete_prefixthey delete_suffixdon't exist contrary to the Ruby documentation.)