Issues compiling nested expression

I am trying to get cel-ruby to compile my expression here - it should work AFAICT. But I'm getting an error..

My test code:

env = Cel::Environment.new(target_list: :list, source_list: :list, required_count: :int)
expression = "size(target_list.filter(e, e in source_list)) >= required_count"

ast = env.compile(expression)

The result:

irb(main):248:0> ast = env.compile(expression)
/Users/james/.rbenv/versions/3.2.1/lib/ruby/gems/3.2.0/gems/cel-0.2.0/lib/cel/checker.rb:398:in `unsupported_operation': unsupported operation (size(target_list.filter(e, e in source_list))) (Cel::CheckError)
	from /Users/james/.rbenv/versions/3.2.1/lib/ruby/gems/3.2.0/gems/cel-0.2.0/lib/cel/checker.rb:300:in `check_standard_func'
	from /Users/james/.rbenv/versions/3.2.1/lib/ruby/gems/3.2.0/gems/cel-0.2.0/lib/cel/checker.rb:140:in `check_invoke'
	from /Users/james/.rbenv/versions/3.2.1/lib/ruby/gems/3.2.0/gems/cel-0.2.0/lib/cel/checker.rb:14:in `check'

Is this a bug? Something unsupported? Am I setting it up wrong?

It is a beautifully designed library otherwise, hope to use it soon 👍🏻