diff --git a/lib/mongo/server/pending_connection.rb b/lib/mongo/server/pending_connection.rb
index 3bb53f4bec..1981e32812 100644
--- a/lib/mongo/server/pending_connection.rb
+++ b/lib/mongo/server/pending_connection.rb
@@ -110,6 +110,24 @@ def handshake_and_authenticate!
 
       private
 
+      # Sends the hello command to the server, then receive and deserialize
+      # the response.
+      #
+      # This method is extracted to be mocked in the tests.
+      #
+      # @param [ Protocol::Message ] Command that should be sent to a server
+      #   for handshake purposes.
+      #
+      # @return [ Mongo::Protocol::Reply ] Deserialized server response.
+      def get_handshake_response(hello_command)
+        @server.round_trip_time_averager.measure do
+          add_server_diagnostics do
+            socket.write(hello_command.serialize.to_s)
+            Protocol::Message.deserialize(socket, Protocol::Message::MAX_MESSAGE_SIZE)
+          end
+        end
+      end
+
       # @param [ BSON::Document | nil ] speculative_auth_doc The document to
       #   provide in speculativeAuthenticate field of handshake command.
       #
@@ -131,12 +149,7 @@ def handshake!(speculative_auth_doc: nil)
         doc = nil
         @server.handle_handshake_failure! do
           begin
-            response = @server.round_trip_time_averager.measure do
-              add_server_diagnostics do
-                socket.write(hello_command.serialize.to_s)
-                Protocol::Message.deserialize(socket, Protocol::Message::MAX_MESSAGE_SIZE)
-              end
-            end
+            response = get_handshake_response(hello_command)
             result = Operation::Result.new([response])
             result.validate!
             doc = result.documents.first
diff --git a/spec/mongo/server/connection_spec.rb b/spec/mongo/server/connection_spec.rb
index a1acf2f69c..3d584ae090 100644
--- a/spec/mongo/server/connection_spec.rb
+++ b/spec/mongo/server/connection_spec.rb
@@ -583,6 +583,28 @@ class ConnectionSpecTestException < Exception; end
       end
     end
 
+    context 'when the server returns unknown saslSupportedMechs' do
+      min_server_version '4.0'
+
+      let(:connection) do
+        described_class.new(server, server.options.merge(connection_pool: pool))
+      end
+
+      before do
+        expect_any_instance_of(Mongo::Server::PendingConnection).to receive(:get_handshake_response).and_wrap_original do |original_method, *args|
+          original_method.call(*args).tap do |result|
+            if result.documents.first.fetch('saslSupportedMechs', nil).is_a?(Array)
+              result.documents.first['saslSupportedMechs'].append('unknownMechanism')
+            end
+          end
+        end
+      end
+
+      it 'does not raise an error' do
+        expect { connection.connect! }.not_to raise_error
+      end
+    end
+
   end
 
   describe '#disconnect!' do