Why am I receiving AWS.EncryptionSDK.AwsEncryptionSdkException "Failed to build header body". when trying to encrypt a string

Using the sample code below I receive the error on the line indicated in the comment below. I am able to encrypt and decrypt using the same key and account via aws kms cli.

public  MemoryStream EncryptMessageWithKMSKey(MemoryStream plaintext, string kmsKeyArn)
{
    var materialProviders =
        AwsCryptographicMaterialProvidersFactory.CreateDefaultAwsCryptographicMaterialProviders();
    var encryptionSdk = AwsEncryptionSdkFactory.CreateDefaultAwsEncryptionSdk();
    var createKeyringInput = new CreateAwsKmsMrkMultiKeyringInput()
    {
        Generator = kmsKeyArn
    };
    var encryptionContext = new Dictionary<string, string>()
    {
        {"purpose", "test"}
    };
    var encryptKeyring = materialProviders.CreateAwsKmsMrkMultiKeyring(createKeyringInput);
    var encryptInput = new EncryptInput
    {
        Plaintext = plaintext,
        Keyring = encryptKeyring,
        EncryptionContext = encryptionContext
    };
    var encryptOutput = encryptionSdk.Encrypt(encryptInput);  // error here !!!!!!!!!!!!!!!!!!!!
    var ciphertext = encryptOutput.Ciphertext;
    return ciphertext;
}