This didn't set well with what I knew of WPA, so I checked Wikipedia: "Pre-shared key mode (PSK, also known as Personal mode) is designed for home and small office networks that don't require the complexity of an 802.1X authentication server. Each wireless network device encrypts the network traffic using a 256 bit key. This key may be entered either as a string of 64 hexadecimal digits, or as a passphrase of 8 to 63 printable ASCII characters."

I just checked a router I maintain, and indeed the administration screen specifies that the WPA password should be 8 to 63 characters.