I get a "Type mismatch". It is expecting String instead of String? But can't understand why

It could be the code is annotated with @NotNull or something like that. Would be strange, given the name, but that's my suspicion. String? Is a nullable string type.

The parameter is not annotated (it is from Guava). The other parameters are. Maybe there is now a package wide NonNull annotation... I will take a look.

If nothing else, changing "String?" to "String" will guarantee that it's not null and will thus make the call to that method unnecessary.