LetsEncrypt certificate returns CertPathValidatorException

Good evening everyone,

I'm trying to do do SSL Pinning for my app and the Lets Encrypt certificate (ISRG Root X1) is returning CertPathValidatorException: Trust anchor for certification path not found.

I'm on Android Studios using Kotlin, using Fuel for the web requests, here is the code I have

fun pinning(ctx: Context): Pair<SSLSocketFactory, KeyStore> {
    val cf: CertificateFactory = CertificateFactory.getInstance("X.509")
    val caInput: InputStream = BufferedInputStream(ctx.resources.openRawResource(R.raw.isrgrootx1))
    val ca: X509Certificate = caInput.use {
        cf.generateCertificate(it) as X509Certificate
    }
    Log.println(Log.DEBUG, "SYSTEM-CA", ca.subjectDN.toString())

    // Create a KeyStore containing our trusted CAs
    val keyStoreType = KeyStore.getDefaultType()
    val keyStore = KeyStore.getInstance(keyStoreType).apply {
        load(null, null)
        setCertificateEntry("ca", ca)
    }

    // Create a TrustManager that trusts the CAs inputStream our KeyStore
    val tmfAlgorithm: String = TrustManagerFactory.getDefaultAlgorithm()
    val tmf: TrustManagerFactory = TrustManagerFactory.getInstance(tmfAlgorithm).apply {
        init(keyStore)
    }

    // Create an SSLContext that uses our TrustManager
    val context: SSLContext = SSLContext.getInstance("TLS").apply {
        init(null, tmf.trustManagers, null)
    }

    return Pair(context.socketFactory, keyStore)
}


val (pin, key) = pinning(this)
FuelManager.instance.socketFactory = pin
FuelManager.instance.keystore = key
Log.println(Log.INFO, "FUEL-MANAGER", "Imported instances successfully")

The FuelManager is run as soon as the app starts (override fun onStart())