Unable to parse input tree, possibly due to dash

Question

Hi I have the following device tree for my board:

&i2c0 {
    status = "okay";

    i2c-mux@77 {
        compatible = "nxp,pca9847";
        reg = <0x77>;
        #address-cells = <1>;
        #size-cells = <0>;

        i2c@1 {
            #address-cells = <1>;
            #size-cells = <0>;
            reg = <0x1>;

            sgtl5000: audio-codec@a {
                #sound-dai-cells = <0>;
                compatible = "fsl,sgtl5000";
                reg = <0xa>;
                VDDA-supply = <&reg_1p8v>;
                VDDIO-supply = <&reg_1p8v>;
                clocks = <&sys_mclk>;
                sclk-strength = <3>;
            };
        };
    };
};

which I don't want to modify because it comes from vendor's BSP. I just want to add an overlay like this:

/dts-v1/;
/plugin/;


&i2c-mux {
    i2c@6 {
        #address-cells = <1>;
        #size-cells = <0>;
        reg = <0x6>;
    
        pmic@5e {
            compatible = "ti,tps65086";
            reg = <0x5e>;
        };
    };
};

However, I'm getting an error:

FATAL ERROR: Unable to parse input tree

It seems that it comes from i2c-mux name which contains dash. However, I don't know how to fix it without modifying the original device tree. Is there a way to add an overlay without touching the original source?