wanna add ability to checking immune to interrupt at this macro

[msli00100]

im using

#showtooltip counterspell
/run if GetSpellCooldown("counterspell")==0 and IsSpellInRange("counterspell","target") and (UnitCastingInfo("target") or UnitChannelInfo("target")) then CastSpellByName("counterspell") end


this macro and i wanna ignore immune to interrupts

i tried insert [if not UnitBuffID("target", 31821)] this words but it didn't work

any1 can help me???

[Stubbze]

i need help too