Prevent UnboundLocalError on empty asn1crypto.x509.IPAddress()
diff --git a/asn1crypto/x509.py b/asn1crypto/x509.py
index 938bb41..9e4550e 100644
--- a/asn1crypto/x509.py
+++ b/asn1crypto/x509.py
@@ -389,6 +389,7 @@
         if self._native is None:
             byte_string = self.__bytes__()
             byte_len = len(byte_string)
+            value = None
             cidr_int = None
             if byte_len in set([32, 16]):
                 value = inet_ntop(socket.AF_INET6, byte_string[0:16])