CAS block error

Unexpected CAS error:

Expected output from:

cb.default_format = :document
cb.set("trans1", "{'karen': '400', 'dipti': '800', 'status':'init' }")=> 360287970189639680
cb.cas("trans1") do |val|
val["new"] = "new"
val
end

Would be {'karen': '400', 'dipti': '800', 'status':'init', 'new': 'new' }

but it returns:

IndexError: string not matched
from (irb):7:in `[]='

This is based on the rdoc example:

c.default_format = :document
c.set("foo", {"bar" => 1})
c.cas("foo") do |val|
  val["baz"] = 2
  val
end
c.get("foo") #=> {"bar" => 1, "baz" => 2}