To detail it, here's what I did:
> listreceivedbyaddress 0 true
"address" : "PtrY1nMip1WXBWif7nd8KkTzyAo4e9gpW3",
"account" : "",
"amount" : 0.00000000,
"confirmations" : 0
> dumpprivkey PtrY1nMip1WXBWif7nd8KkTzyAo4e9gpW3
I pasted it to the Wallet Details tab of this site:https://www.bitaddress.org
then it says:
The text you entered is not a valid Private Key!
Would you like to use the entered text as a passphrase and create a Private Key using a SHA256 hash of the passphrase?