could this be: "you must be dreaming"
In English, yes, it means the same as you are surely dreaming.
Perhaps the more literal way was preferred as it still reflects the meaning and it helps with comprehending the usage of words like sigur.
Or even, more colloquially, 'you've got to be dreaming!'.
Suddenly and without warning, Duo discovers the present progressive tense!
Is sigur not also Certainly?