Starlark dictionary accepting NoneType for values?

[agoulti]

Is Starlark dictionary meant to accept None values?

I've added a field of type Type.STRING_DICT and the following definition:

exec_properties = {
"container-image": "docker://gcr.io/gcp-runtimes/ubuntu_16_0_4@sha256:096632d8fb3e78fbd58ae6a2b25ed46020dc70e65d89bca774af6f7b2de6898c",
"OSFamily": None,
}

I get the following message:
ERROR: /usr/local/google/home/agoulti/examples/toolchain-resolution-example/BUILD:13:1: //:plain_platform: expected value of type 'string' for dict value element, but got None (NoneType)

[agoulti]

Got a clarification: this is not a language limitation, but the type. STRING_DICT does not accept Nulls. Part of the reason is that there are many different representations: --output={build|xml|proto}, which all need to have representations.