This is correct.
(DC) + (DB) = D(C + B)
Your choices are to;
- ignore the fault, (input > 9) . x= don't care? or are there consequences?
- display a fault ( error detect ) is it useful?
- blank the display with flashing Dot or something( from erroneous data )
- use a HEX LED converter and show result as a,b,c,d,e,f ( is that useful? )
Only you can tell if there are consequences of a fault condition but often fault detection is a good thing if you do something with it. Sometimes this is only done on a power On reset with JTAG tests. The higher the % fault detection (metric for Test Engineers, the better the design.