Cannot set HttpApi Event Type on AWS::Serverless::StateMachine

I'm attempting to configure step functions to be triggered from an HttpApi (rather than a REST api). Here is my template:

```
APIGatewayDeployment:
Type: AWS::Serverless::HttpApi
Properties:
StageName: !Ref pStage
DefinitionBody:
info:
title: !Ref AWS::StackName
version: "1.0"
openapi: 3.0.1

MyStateMachine:
Type: AWS::Serverless::StateMachine
Properties:
DefinitionUri: statemachines/MyStateMachine.asl.json
DefinitionSubstitutions:
MyFunction: !GetAtt MyFunction.Arn
Events:
HttpAPIGateway:
Type: HttpApi
Properties:
Path: /inbound
Method: post
ApiId: !Ref APIGatewayDeployment
Policies:
- LambdaInvokePolicy:
FunctionName: !Ref MyFunction
```

This builds, but fails validation or deployment with the following:
`Error: Failed to create changeset for the stack: , ex: Waiter ChangeSetCreateComplete failed: Waiter encountered a terminal failure state Status: FAILED. Reason: Transform AWS::Serverless-2016-10-31 failed with: Invalid Serverless Application Specification document. Number of errors found: 1. Resource with id [MyStateMachine] is invalid. Event with id [HttpAPIGateway] is invalid. Resource dict has missing or invalid value for key Type. Event Type is: HttpApi.`

If I've done something wrong, let me know! I've been unable to find helpful documentation about using HTTP rather than REST with step functions.