isblank & vlookup formula returns #n/a result and incorrect result

I am building a spreadsheet to autopopulate cells if certain criteria is filled in. Using this formula works for 2 years through 5 years, but produces a #n/a result for the 1 year and outputs an incorrect result for 6 months. I've tried using the 'isblank' and 'istext' formulas. Any help would be greatly appreciated.

=IF(ISBLANK(C3),"",VLOOKUP(A3,DropDownLists!A$2:B$7,2)+C3)

Term Term / End Date Lookup Calculation
6 months 183
1 year 365
2 years 730
3 years 1095
4 years 1460
5 years 1825