diff --git a/lib/hiera/backend/eyaml/encryptors/pkcs7.rb b/lib/hiera/backend/eyaml/encryptors/pkcs7.rb
index 95fbf0e..f17e5f9 100644
--- a/lib/hiera/backend/eyaml/encryptors/pkcs7.rb
+++ b/lib/hiera/backend/eyaml/encryptors/pkcs7.rb
@@ -43,6 +43,8 @@ def self.encrypt(plaintext)
               public_key_rsa = OpenSSL::PKey::RSA.new(public_key_pem)
               public_key_x509 = OpenSSL::X509::Certificate.new
               public_key_x509.public_key = public_key_rsa.public_key
+            else
+              raise StandardError, "file #{public_key_pem} cannot be used to encrypt - invalid public key format"
             end
 
             cipher = OpenSSL::Cipher.new('aes-256-cbc')