melt fails when pattern not found

[MichaelChirico]

DT = data.table(id = 1:3, a1 = rnorm(10), a2 = rnorm(10))
melt(DT, id.vars = 'id', measure.vars = patterns(a = 'a', b = 'b'))

Error in melt.data.table(DT, id.vars = "id", measure.vars = patterns(a = "a", :
Unsupported type 'NULL'

Quite a confusing error