`list` with @sparse trait fails code generation with trait `AsRef<str>` is not implemented for `Option<Type>`

[diegotry]

I have a list definition:

@sparse
list ExecutionStatusList {
    member: ExecutionStatus
}

where execution status is an enum:

enum ExecutionStatus {
    RUNNING
    SUCCEEDED
    FAILED
    TIMED_OUT
    STOPPED
}

when I tried to build, it fails with:

error[E0277]: the trait bound `Option<ExecutionStatus>: AsRef<str>` is not satisfied
   --> amzn-(..)client/src/operation/list_(...).rs:203:97
    |
203 | ...                   query.push_kv("StatusFilter", &::aws_smithy_http::query::fmt_string(inner_5));
    |                                                      ------------------------------------ ^^^^^^^ the trait `AsRef<str>` is not implemented for `Option<ExecutionStatus>`
    |                                                      |
    |                                                      required by a bound introduced by this call 

My assumption is that, when I moved to sparse lists (which accepts 'null'/'Optional'), the generated code didn't do well with it.