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here is what Line 8a says: 

 

Total social security wages and tips (total of boxes 3 and 7 on Form(s) W-2)
and railroad retirement (tier 1) compensation. If $147,000 or more, skip lines
8b through 10, and go to line 11

 

did your wife's SS wages and tips exceed $147,000? then box 8a would be blank by design.