Kr-33

I am confused by the usage "Kr-33" in the main article on Krypton:

In 1960 it was internationally agreed that the fundamental unit of length, the metre, should be defined as 1 m = 1,650,763.73 wavelengths (in vacuo) of the orange-red line of Kr-33.

What does the 33 refer to?

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It is a typo; it should be

It is a typo; it should be Kr-86.

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