LetsEncrypt certificate returns CertPathValidatorException - kotlin

Question

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())