multiindex key definition - footgun

I am having an issue with CW-storage-plus. (thanks to Eris/Phil @0xPhilipp for pointing it out)
I have managed to produce an example which demonstrates it here - [testcase](https://github.com/PFC-developer/storage-testcase)

I have a key which is a (u64,&str) .. and it works ok, until I put in a u64 > 128, and it then gives a UTF-8 error

how do I remedy this.

is there a way to do this without migrating the index in the existing smart contract
the error returned is as follows
```
called `Result::unwrap()` on an `Err` value: InvalidUtf8 { msg: "invalid utf-8 sequence of 1 bytes from index 9" }
thread 'testing::tests::test_128' panicked at contracts/hub-tf/src/testing/tests.rs:2100:10:
```

this was due to a multiindex signature not being defined properly.
the following patch to the test case [resolves](https://github.com/PFC-developer/storage-testcase/compare/main...mod-2) it.

the issue is here as I think this should have resulted in a compile error (the PK was badly defined in the original case), but it wasn't.

Thanks!

Start with the linked storage-testcase repository and the panic location at contracts/hub-tf/src/testing/tests.rs:2100, then compare the original case with the mod-2 patch. Trace how the malformed multiindex signature is accepted and determine what behavior would prevent the invalid UTF-8 failure. Done means the bad primary-key definition is rejected or diagnosed before runtime.

Written by the indexing model from the issue text.