iOS SSL Pinning index beyond bounds

Question

Sometime I'm getting crashes from firebase: Index 1 beyond bounds [0..0] but in my app I'm checking if serverTrust contains any certificates. Any idea why? position 0 is leaf, position 1 is intermediate and 2 is root. here is code:

 guard challenge.protectionSpace.authenticationMethod == NSURLAuthenticationMethodServerTrust else {
        return false
    }

    guard let serverTrust = challenge.protectionSpace.serverTrust else {
        return false
    }

    var secResult: SecTrustResultType = .invalid
    let status = SecTrustEvaluate(serverTrust, &secResult)

    guard status == errSecSuccess else {
        return false
    }

    let position = 1

    if SecTrustGetCertificateCount(serverTrust) < 1 { return false }

    guard let rootCertificate = SecTrustGetCertificateAtIndex(serverTrust, position) else {
        return false
    }

    do {
        let localCert = try Certificate.cert()
        let serverCert = SecCertificateCopyData(rootCertificate) as Data
        return serverCert == localCert
    } catch {
        return false
    }