diff --git a/bip-0075.mediawiki b/bip-0075.mediawiki
index 9babf89..878d708 100644
--- a/bip-0075.mediawiki
+++ b/bip-0075.mediawiki
@@ -141,7 +141,7 @@ message ProtocolMessage {
     required ProtocolMessageType message_type = 3;
     required bytes serialized_message = 4;
     optional string status_message = 5;
-    optional bytes identifier = 6;
+    required bytes identifier = 6;
 }
 </pre>
 
@@ -177,7 +177,7 @@ message EncryptedProtocolMessage {
     required bytes receiver_public_key = 5;
     required bytes sender_public_key = 6;
     required uint64 nonce = 7;
-    optional bytes identifier = 8;
+    required bytes identifier = 8;
     optional string status_message = 9;
     optional bytes signature = 10;
 }
diff --git a/bip-0075/paymentrequest.proto b/bip-0075/paymentrequest.proto
index 5a08192..cb04369 100644
--- a/bip-0075/paymentrequest.proto
+++ b/bip-0075/paymentrequest.proto
@@ -70,7 +70,7 @@ message ProtocolMessage {
     required ProtocolMessageType message_type = 3;  // Message Type of serialized_message
     required bytes serialized_message = 4;          // Serialized Payment Protocol Message
     optional string status_message = 5;             // Human-readable Payment Protocol status message
-    optional bytes identifier = 6;                  // Unique key to identify this entire exchange on the server. SHA256 of initial serialized InvoiceRequest SHOULD be used by default
+    required bytes identifier = 6;                  // Unique key to identify this entire exchange on the server. SHA256 of initial serialized InvoiceRequest SHOULD be used by default
 }
 
 message EncryptedProtocolMessage {
@@ -81,7 +81,7 @@ message EncryptedProtocolMessage {
     required bytes receiver_public_key = 5;         // Receiver's DER-encoded EC Public Key
     required bytes sender_public_key = 6;           // Sender's DER-encoded EC Public Key
     required uint64 nonce = 7;                      // Microseconds since epoch
-    optional bytes identifier = 8;                  // Unique key to identify this entire exchange on the server. SHA256 of initial serialized InvoiceRequest SHOULD be used by default
+    required bytes identifier = 8;                  // Unique key to identify this entire exchange on the server. SHA256 of initial serialized InvoiceRequest SHOULD be used by default
     optional string status_message = 9;             // Human-readable Payment Protocol status message
     optional bytes signature = 10;                   // Signature over the full EncryptedProtocolMessage with EC Key Belonging to Sender / Receiver, respectively
 }
\ No newline at end of file