diff --git a/samples/gui/wasm-runtime-wgl/src/platform/zephyr/iwasm_main.c b/samples/gui/wasm-runtime-wgl/src/platform/zephyr/iwasm_main.c
index 6c42acc8..48681550 100644
--- a/samples/gui/wasm-runtime-wgl/src/platform/zephyr/iwasm_main.c
+++ b/samples/gui/wasm-runtime-wgl/src/platform/zephyr/iwasm_main.c
@@ -69,6 +69,9 @@ static bool host_init()
 
 int host_send(void * ctx, const char *buf, int size)
 {
+    if (!uart_dev)
+        return 0;
+
     for (int i = 0; i < size; i++)
         uart_poll_out(uart_dev, buf[i]);
 
diff --git a/samples/gui/wasm-runtime-wgl/zephyr-build/prj.conf b/samples/gui/wasm-runtime-wgl/zephyr-build/prj.conf
index 2375969e..79d89205 100644
--- a/samples/gui/wasm-runtime-wgl/zephyr-build/prj.conf
+++ b/samples/gui/wasm-runtime-wgl/zephyr-build/prj.conf
@@ -5,3 +5,5 @@ CONFIG_PRINTK=y
 CONFIG_LOG=y
 #CONFIG_UART_2=y
 CONFIG_UART_INTERRUPT_DRIVEN=y
+CONFIG_STACK_SENTINEL=y
+CONFIG_MAIN_STACK_SIZE=2048
diff --git a/samples/littlevgl/vgl-wasm-runtime/src/platform/zephyr/iwasm_main.c b/samples/littlevgl/vgl-wasm-runtime/src/platform/zephyr/iwasm_main.c
index 1fc3eb94..64e27d71 100644
--- a/samples/littlevgl/vgl-wasm-runtime/src/platform/zephyr/iwasm_main.c
+++ b/samples/littlevgl/vgl-wasm-runtime/src/platform/zephyr/iwasm_main.c
@@ -55,7 +55,7 @@ static bool host_init()
     uart_dev = device_get_binding(HOST_DEVICE_COMM_UART_NAME);
     if (!uart_dev) {
         printf("UART: Device driver not found.\n");
-        return;
+        return false;
     }
     uart_irq_rx_enable(uart_dev);
     uart_irq_callback_set(uart_dev, uart_irq_callback);
@@ -64,6 +64,9 @@ static bool host_init()
 
 int host_send(void * ctx, const char *buf, int size)
 {
+    if (!uart_dev)
+        return 0;
+
     for (int i = 0; i < size; i++)
         uart_poll_out(uart_dev, buf[i]);
 
diff --git a/samples/littlevgl/vgl-wasm-runtime/zephyr-build/prj.conf b/samples/littlevgl/vgl-wasm-runtime/zephyr-build/prj.conf
index 2375969e..79d89205 100644
--- a/samples/littlevgl/vgl-wasm-runtime/zephyr-build/prj.conf
+++ b/samples/littlevgl/vgl-wasm-runtime/zephyr-build/prj.conf
@@ -5,3 +5,5 @@ CONFIG_PRINTK=y
 CONFIG_LOG=y
 #CONFIG_UART_2=y
 CONFIG_UART_INTERRUPT_DRIVEN=y
+CONFIG_STACK_SENTINEL=y
+CONFIG_MAIN_STACK_SIZE=2048