If two cells are blank then return blank, else vlookup cell with populated data

Need a hand .. I'm stumped and don't know why:

Existing formula IF(ISBLANK(D4:D5)," ",IFERROR((IF(ISBLANK(D4),VLOOKUP(D5,'Master List'!B:O,14,0),VLOOKUP(D5,'Master List'!D:O,12,0))),"Not TSRd"))

1. If D4 and D5 are empty, then in D6 return blank
2. If d4 is not empty, vlookup d4 on master list and return TSR date in D6
3. if d5 is not empty, vlookup d5 on master list and return TSR date in D6
4. If D4 or D5, when entered are not found in the master list, return "Not TSRd"

Thank you in advance for your input!