Mocking functions with every fails in Android unit tests on JWT

I have the following data class with its functions:

@JsonClass(generateAdapter = true)
data class AuthToken(
    @Json(name = "token") val access: String,
    @Json(name = "refreshToken") val refresh: String
) {
    fun isAccessTokenValid(): Boolean = isTokenValid(access)

    fun isRefreshTokenValid(): Boolean = isTokenValid(refresh)

    private fun isTokenValid(token: String): Boolean {
        return try {
            val jwt = JWT(token)
            val isExpired = jwt.isExpired(0)
            Timber.d("Token ${if (isExpired) "expired" else "valid"} [${jwt.details()}]")
            !isExpired
        } catch (e: DecodeException) {
            Timber.w(e, "Invalid auth token.")
            false
        }
    }
}

In my unit tests I have the following lines:

val authToken = AuthToken("accessToken", "refreshToken")
every { authToken.isAccessTokenValid() } returns true
every { authToken.isRefreshTokenValid() } returns false

But I get the following error:

Missing mocked calls inside every { ... } block: make sure the object inside the block is a mock.

On line val jwt = JWT(token)

Why does the isTokenValid runs at all when I'm trying to mock the outer functions calling it?

Any idea how to solve it?

To stub object's methods you have to create it as a mockk:

val authToken = mockk<AuthToken>()