Why can't I perform arithmetic on an integer?

Output:

18:49:43.694 - Workspace.TModule:4: attempt to perform arithmetic on local 'a' (a table value)
18:49:43.694 - Stack Begin
18:49:43.694 - Script 'Workspace.TModule', Line 4 - method GeneratePlate

Source Code:

--continued, but this is the only part that errors.
function tm.GeneratePlate(a,b,c,type,x,y,z)
	if a%2~=0 then
		a=a-1
	end 
--continued, but this is the only part that errors.

Running Code:

tm:GeneratePlate(10,math.random(50,200),10,math.random(0,17),math.random(-200,200),0,math.random(-200,200))

I don't understand what's wrong with anything. All of this should work, I looked over it again and again.