Can't use the character '_' in the .overlay file.

Question

I tried to write the following simple '.overlay' file:

// To get started, press Ctrl+Space to bring up the completion menu and view the available nodes.

// You can also use the buttons in the sidebar to perform actions on nodes.
// Actions currently available include:

// * Enabling / disabling the node
// * Adding the bus to a bus
// * Removing the node
// * Connecting ADC channels

// For more help, browse the DeviceTree documentation at https://docs.zephyrproject.org/latest/guides/dts/index.html
// You can also visit the nRF DeviceTree extension documentation at https://docs.nordicsemi.com/bundle/nrf-connect-vscode/page/guides/ncs_configure_app.html#devicetree-support-in-the-extension
&gpio0 {
	status = "okay";
};

&gpiote {
	status = "okay";
};


/ {
	leds {
		compatible = "gpio-leds";
		yellow_led: led21 {
			label = "led";
			gpios = <&gpio0 6 GPIO_ACTIVE_HIGH>;
		};
	};

	aliases {
		yellow_led = &yellow_led;
	};
};

However, I am getting the following error:

devicetree error: /aliases: alias property name 'yellow_led' should include only characters from [0-9a-z-]
CMake Error at /home/ams/ncs/v2.9.0/zephyr/cmake/modules/dts.cmake:295 (execute_process):

but if you look inside the .dts files provided by the 'nrf52dk' I can see the following node:

	pwmleds {
		compatible = "pwm-leds";
		pwm_led0: pwm_led_0 {
			pwms = <&pwm0 0 PWM_MSEC(20) PWM_POLARITY_INVERTED>;
		};
	};

so why it's allowed in the .dts file of the board but not in the .overlay file?