PICkit 2 not detecting target Vdd (jodyflorian)

I'm using a non-clone PICkit2, programming a PIC16F57 with ICSP. It works when I use the PICkit 2's power, but not when I use an external 5v source. If I isolate the circuit from everything (except the RC clock circuit) then go to "Tools" > "Target Vdd So..