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What happens if you remove the `!` from the end of `clusterName`?

the error is the same, this line is a cause
<https://github.com/pulumi/pulumi-aws/blob/master/sdk/nodejs/cloudwatch/metricAlarm.ts#L234>

I found a workaround using `interpolate` but it's not great IMO

I believe what you are running into here is the same issue tracked in <https://github.com/pulumi/pulumi/issues/6175>.  Note that this actually works fine in Node.js, just the TypeScript types don't allow it - so you can just cast to `any` and it will work.  But there is also an improvement at the TypeScript layer being pursued in <https://github.com/pulumi/pulumi/pull/6323> which would make this work without any need for workarounds.