diff --git a/test/gatt_client/Makefile b/test/gatt_client/Makefile
index d639a6e99..3d6a5ade5 100644
--- a/test/gatt_client/Makefile
+++ b/test/gatt_client/Makefile
@@ -4,7 +4,7 @@ CC = g++
 
 BTSTACK_ROOT =  ../..
 
-CFLAGS  = -DUNIT_TEST -x c++ -g -Wall -I. -I../ -I${BTSTACK_ROOT}/src -I${BTSTACK_ROOT}/ble -I${BTSTACK_ROOT}/include
+CFLAGS  = -DUNIT_TEST -x c++ -g -Wall -Wnarrowing -I. -I../ -I${BTSTACK_ROOT}/src -I${BTSTACK_ROOT}/ble -I${BTSTACK_ROOT}/include
 LDFLAGS +=  -lCppUTest -lCppUTestExt
 
 VPATH += ${BTSTACK_ROOT}/ble 
diff --git a/test/gatt_client/mock.c b/test/gatt_client/mock.c
index e18476e83..a4a94766b 100644
--- a/test/gatt_client/mock.c
+++ b/test/gatt_client/mock.c
@@ -24,7 +24,7 @@ uint16_t get_gatt_client_handle(void){
 }
 
 void mock_simulate_command_complete(const hci_cmd_t *cmd){
-	uint8_t packet[] = {HCI_EVENT_COMMAND_COMPLETE, 4, 1, cmd->opcode & 0xff, cmd->opcode >> 8, 0};
+	uint8_t packet[] = {HCI_EVENT_COMMAND_COMPLETE, 4, 1, (uint8_t) (cmd->opcode & 0xff), (uint8_t) (cmd->opcode >> 8), 0};
 	registered_l2cap_packet_handler(NULL, HCI_EVENT_PACKET, NULL, (uint8_t *)&packet, sizeof(packet));
 }