View the original community article here
Last tested: Aug 14, 2018
This warning message comes up when there is a character mismatch between the field being filtered and the values that are entered in it, like so:
One "gotcha" can be if the values were copied and pasted from another place like gmail -- there could be some weird character formatting in that carries over even if you can't see it visually. Verify that you're using a plain text editor or csv to source the values you paste in.