What is a AT89Cx051 chip supposed to behave like during verify with both lock bits set?

My programmer (described earlier) can programme, erase, verify etc. with no problems. Then I decided to add a "programme lock bits" function. After setting both lock bits verify fails (as expected), but "blank check" completes with no errors. Is the chip supposed to "look" blank?

But I thought that was ok. But then it turns out the chips behave very much like blank chips after programming the lock bits... If I skip that step the chip works fine in circuit.

Any ideas? Does anybody have a better description of the "programme lock bits" procedure than what's in the data sheet?

Best regards,

Mikkel C. Simonsen