Help with Chassis No.

Hi, Peeps :)
So my lovely caravan came home today and I'm thrilled to bits with her!  I'm trying to insure her now and need  the chassis no so I took a look at the little silver plate by the door and found the following chassis no.:  89CU1724384

However, the form I'm filling in says that it's not in the right format and I'm at a loss what could be wrong with it.
She's a Sprite Musketeer if that makes any difference.  What am I doing wrong?