Spring wrongfully tries to cast my sealed class into a Collection. Why? - spring

Question

I have a problem with Spring, kotlin, and sealed classes, and don't know where it comes from. It would seem Spring tries to cast my ErrorDto class into a collection, when it shouldn't have to cast it.
Context
I have a sealed class Response that wraps my responses.
sealed interface Response<T>
class ErrorResponse<T>(status: HttpStatus, message: String) : Response<T>,
ResponseEntity<ErrorDto>(ErrorDto(status.value(), message), status)
class SuccessResponse<T>(responseEntity: ResponseEntity<T>) : Response<T>, ResponseEntity<T>(responseEntity.body, responseEntity.headers, responseEntity.statusCode) {
constructor(body: T): this(ok(body))
}
class ErrorDto(val status: Int, #SerializedName("error") val message: String? = null)
All my requests return types are Response<AType>, where AType is a typical response type so that:
we have a ResponseEntity<AType> when the request succeeds,
and when the request encounters an error, it returns a ResponseEntity<ErrorDto>
The wrapper Response is here so that I can correctly type the responses to the REST requests.
Working example
This works correctly with custom types, and here is an example of request that works at runtime:
#GetMapping("/test1")
fun test1(): Response<MyCustomType> {
val response: Response<MyCustomType> = if (true) {
ErrorResponse(HttpStatus.CONFLICT, "test")
} else {
SuccessResponse(MyCustomType())
}
return response
}
The problem
And now, here's an example of what doesn't work at runtime (it compiles just fine):
#GetMapping("/test2")
fun test2(): Response<List<String>> {
val response: Response<List<String>> = if (true) {
ErrorResponse(HttpStatus.CONFLICT, "test")
} else {
SuccessResponse(listOf("toto"))
}
return response
}
The runtime error with this test2 is:
[org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: class ErrorDto cannot be cast to class java.util.Collection
Why?
It would seem that Spring tries to cast the type to a collection when the wrapped response type is a List (Response<List<String>> here), when it shouldn't do that.
Any idea where this problem comes from and why Spring tries to cast to a Collection when it shouldn't do that?
N.B.: As a workaround, I wrapped my List with MyCustomClass in order to go around the problem, but I would still like to understand the why.