Collected in the Brothers Grimm, the story of Rumplestiltskin revolves around a wager to guess the name of an imp, in order to escape an earlier bargain.
Rumplestiltskin has nothing to gain by offering this wager. Why do it?
Perhaps because the original bargain is already illegitimate? -- because you cannot have a proper contract with the nameless?
But if this happens to you, don't guess Rumplestiltskin right away. Consider some of the variants.
Rumplestiltskin is taxonomized as Aarne-Thompson #500: the Name of the Helper.