envValue does not accept int

[bwbioinfo]

Trying to inject an integer into the environment from an input value yields the following:

'envValue expression must evaluate to a str. Got '6' for expression '$(inputs.threads)'.

any minimal .cwl that has includes the following would produce the same error:

requirements:
- class: EnvVarRequirement
  envDef: 
    THREADS: $(inputs.threads)

inputs:
  threads:
    doc: |-
      The number of parallel iterations.
    type: string
    default: 4

As it is "threads" it should not accept anything other than an integer. I can think of a JS workaround converting it to a string but why exactly can't this be other flat atomic values?
I confirmed that it is indeed only accepting string inputs here:
https://www.commonwl.org/v1.0/CommandLineTool.html#EnvVarRequirement

cwltool version: 3.1.20211107152837

[mr-c]

I think accepting integers here is an easy thing to do for CWL v1.3