POST /api/v2/flows/executions 400 Bad Request on valid flowId

Trying to trigger a flow via the .NET SDK but getting a 400. The docs say I just need the flowId and inputs.

var executionRequest = new FlowExecutionRequest { FlowId = “abc-123”, Inputs = new Dictionary<string, object> { { “name”, “test” } } };
var result = await apiClient.PostAsync(“/api/v2/flows/executions”, executionRequest);

The flow exists and is published. Why is it failing?