fetch() returns : warning: block supersedes default value argument

fetch() returns : warning: block supersedes default value argument

Why do I get this warning and what it means?
a = [1,3,5]
a.fetch(0) #=> 1
a.fetch(0,0) #=> 1
a.fetch(22) { "out of range" } #=> out of range
a.fetch(0,0) { "out of range" } #=> returns 1 with warning: block
supersedes default value argument