merge() method is broken?

[key88sf]

Ruby 3.04:

3.0.4 :001 > RRR = ImmutableStruct.new(:source_type, :text, :score, :data, :error_message)
 => RRR 
3.0.4 :002 > x1 = RRR.new(text: "Hello")
 => #<struct RRR source_type=nil, text="Hello", score=nil, data=nil, error_message=nil> 
3.0.4 :003 > x2 = x1.merge(score: 99)
(irb):3:in `<main>': undefined method `merge' for #<struct RRR source_type=nil, text="Hello", score=nil, data=nil, error_message=nil> (NoMethodError)