diff --git a/EPD/EPD_service.c b/EPD/EPD_service.c
index 7f34a5c..2b5439e 100755
--- a/EPD/EPD_service.c
+++ b/EPD/EPD_service.c
@@ -661,6 +661,19 @@ static void dispatch_command(ble_epd_t* p_epd, uint16_t command_16bit,
         case CMD_FIRMWARE_VERSION:
             handle_firmware_version(p_epd);
             break;
+        case CMD_DIRECT_WRITE_PARTIAL_START:
+        case CMD_BUZZER_ACTIVATE:
+            // Commands the client sends and blocks on, but this firmware does
+            // not implement. Reply with the standard 4-byte NACK frame
+            // {0xFF, cmd, error, 0x00} so the client fails fast instead of
+            // waiting for an ACK that never comes and timing out.
+            {
+                NRF_LOG_WARNING("Unsupported command 0x%04X; sending NACK", command_16bit);
+                uint8_t nack[] = {0xFF, (uint8_t)(command_16bit & 0xFF),
+                                  RESP_ERR_UNSUPPORTED_CMD, 0x00};
+                (void)send_response(p_epd, nack, sizeof(nack));
+            }
+            break;
         default:
             NRF_LOG_DEBUG("Unknown command: 0x%04X", command_16bit);
             break;
diff --git a/EPD/EPD_service.h b/EPD/EPD_service.h
index 7ff82ef..b1eee0b 100755
--- a/EPD/EPD_service.h
+++ b/EPD/EPD_service.h
@@ -74,6 +74,9 @@ void ble_epd_evt_handler(ble_evt_t const* p_ble_evt, void* p_context);
 
 #define CMD_LED_ACTIVATE        0x0073
 
+#define CMD_DIRECT_WRITE_PARTIAL_START  0x0076  // Not implemented on nRF; client waits for a response
+#define CMD_BUZZER_ACTIVATE             0x0077  // Not implemented on nRF; client waits for a response
+
 #define CMD_REBOOT              0x000F
 #define CMD_FIRMWARE_VERSION    0x0043
 #define CMD_READ_MSD            0x0044
@@ -101,6 +104,7 @@ void ble_epd_evt_handler(ble_evt_t const* p_ble_evt, void* p_context);
 #define RESP_DIRECT_WRITE_REFRESH_SUCCESS 0x73
 #define RESP_DIRECT_WRITE_REFRESH_TIMEOUT 0x74
 #define RESP_DIRECT_WRITE_ERROR      0xFF
+#define RESP_ERR_UNSUPPORTED_CMD     0x01  // NACK error code for commands this firmware does not implement
 #define RESP_LED_ACTIVATE_ACK        0x73
 #define RESP_FIRMWARE_VERSION  0x43
 #define RESP_MSD_READ          0x44