mux warning "Clock multiplexers are found and protected"

I implemented a simple mux like this

always @ (sel or in1 or in2 or in3 or in4) 
begin
    case(sel)
        2'b00 : out = in1; 
        2'b01 : out = in2;
        2'b10 : out = in3;
        default : out = in4;
    endcase
end

Is working as supposed to, but Quartus gives me this warning:

Warning: Clock multiplexers are found and protected

Warning: Found clock multiplexer inmux:inst6|Mux0

What is this and how (should I?) modify code to avoid?

Thanks in advance,