Shouldn't it be "a prank"? Or in this case, can "a" be skipped?
In this case, you don't have to have the indefinite article "a" before the word "prank" because the preposition "of" in the sentence takes on the object and shows that it is a part of something that has to do with a "prank" that is some kind of "prank" not specified. Some type of prank implies that the word "prank" is plural here, since the prank could be a prank from all possible pranks. In many sentences that follow the preposition "of" or refer to a particular type "of" something, the object often takes on a hidden plural form and the article can be dropped. Ex: What type of ice cream do you want? Ex: Is this some kind of joke? Ex: I want to buy some type of car that can go off-road.