Ligation Product is Shoter than Vector

[ma]

Dear netters:
At present, I clone a PCR product. When I ligate the PCR product into pGEM7Z vector, I got unusual result.
It should have two kind products. One is vector plus insert, one is vector alone. But I got the third kind
product, its size is shorter than vector after a single enzyme cut (say HindIII, which has unique site in
pGEM7Z). I wonder the reason. Anyone have this kind of experience and kindly give some information.
Ma