Dictionary parameter in ODataActionParameters

ODataActionParameters does not seem to support dictionaries in a complex parameter. Here's a minimal example to reproduce it:

public class Entity
{
    public string Id { get; set; }
}

public class ComplexParam
{
    public string StringValue { get; set; }

    public Dictionary<string, string> EnvVars { get; set; }
}

private IEdmModel GetModel()
{
    var builder = new ODataConventionModelBuilder();
    var entity = builder.EntitySet<Entity>("Entities");
    entity.EntityType.HasKey(x => x.Id);

    var action = entity.EntityType.Collection.Action("TestAction");
    action.Parameter<ComplexParam>("ComplexParam").Required();

    return builder.GetEdmModel();
}

The controller is as following:

[ODataRoutePrefix("Entities")]
public class EntityController : ODataController
{
    [ODataRoute("TestAction")]
    [HttpPost]
    public void TestAction(ODataActionParameters parameters)
    {
        var obj = parameters?["ComplexParam"];
        if(obj is ComplexParam cp)
        {
            // do something
        }
    }

The request I send:

{
    "ComplexParam":  {
        "StringValue": "1",
        "EnvVars": {
            "a": "b",
            "c": "d"
        }
    }
}

parameters is always null in the controller. If I remove the dictionary property and the EnvVars json field from the request, it works as expected. I tried with both Dictionary<string, string> and Dictionary<string, object> and neither works, is there any workaround?

Start with the ODataConventionModelBuilder setup and EntityController.TestAction shown in the issue, then reproduce the request with the dictionary property present and absent. Trace how the ODataActionParameters value is bound for the complex parameter; done means the request populates parameters and preserves the EnvVars dictionary, with the workaround or limitation documented if support is not possible.