NEW YORK (TheStreet) -- Shares of Janus Capital (JNS)  surged Friday after Bill Gross announced he would leave PIMCO High Income Fund  (PHK) - Get Report , the firm he founded, for Janus. TheStreet's Jim Cramer says he understand this because Gross is such a huge name who raised $2 trillion in bonds.

Cramer notes some sources say Gross would have been fired tomorrow, and so Gross got out ahead of this development.

Cramer thinks a lot of people are shorting bond funds and are trying to short high-yield, but he advises investors to be careful. If you are shorting a closed end fund, then Cramer says to make sure it is trading well above the net asset value because there will be pullouts of bond selling, not of individuals. He notes individuals will not have to sell bonds because of redemptions, but the net asset value can go down.

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Cramer warns these funds are not as vulnerable as investors might think. He advises investors stay out of this game altogether, but those who get involved should go after those that have net asset value well below the prices.

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