Getting Microsoft.Azure.WebJobs.Host: Can't bind parameter 'data' to type 'System.String'

I have a Durable Azure Function where I'm trying to call an Activity with a custom object parameter (it should be automatically JSON serialized and deserialized), but I'm getting this error:

The 'CreateStagingSchema' function is in error: Microsoft.Azure.WebJobs.Host: Error indexing method 'Functions.CreateStagingSchema'. Microsoft.Azure.WebJobs.Host: Can't bind parameter 'data' to type 'System.String'

Here is the calling code:

public sealed record ActivityInput { public Guid AccountId { get; init; } ... all simple types } var activityInput = new ActivityInput { AccountId = accountId, ... }; await context.CallActivityAsync(nameof(CreateStagingSchema), activityInput);

And the Activity function is:

[Function(nameof(CreateStagingSchema))] public async Task CreateStagingSchema( [ActivityTrigger] ActivityInput data, FunctionContext executionContext) { ... }