diff --git a/lib/ably/models/protocol_message.rb b/lib/ably/models/protocol_message.rb
index 0088fded..19ac1662 100644
--- a/lib/ably/models/protocol_message.rb
+++ b/lib/ably/models/protocol_message.rb
@@ -249,7 +249,6 @@ def attributes
     # Return a JSON ready object from the underlying #attributes using Ably naming conventions for keys
     def as_json(*args)
       raise TypeError, ':action is missing, cannot generate a valid Hash for ProtocolMessage' unless action
-      raise TypeError, ':msg_serial is missing, cannot generate a valid Hash for ProtocolMessage' if ack_required? && !has_message_serial?
 
       attributes.dup.tap do |hash_object|
         hash_object['action']   = action.to_i
diff --git a/spec/unit/models/protocol_message_spec.rb b/spec/unit/models/protocol_message_spec.rb
index b279b3c5..32c74cbc 100644
--- a/spec/unit/models/protocol_message_spec.rb
+++ b/spec/unit/models/protocol_message_spec.rb
@@ -397,14 +397,6 @@ def new_protocol_message(options)
       end
     end
 
-    context 'with missing msg_serial for ack message' do
-      let(:model) { new_protocol_message({ :action => message_action }) }
-
-      it 'it raises an exception' do
-        expect { model.to_json }.to raise_error TypeError, /msg_serial.*missing/
-      end
-    end
-
     context 'is aliased by #to_s' do
       let(:model) { new_protocol_message({ :action => attached_action, :channelSerial => 'unique', messages: [message1, message2, message3], :timestamp => as_since_epoch(Time.now) }) }