why "Singularity in Average Information Matrix"? how to do?

Dear friends:

         I want to estimate the heritability of fish body weight. The model
is simple:

Data analysis for Flounder

 animal !A !P

 sire !A !P

 dam !A !P

 tank * !I

 age

 bl

 wt

yaping.dat !ALPHA !SKIP 1 !MAKE

yaping.dat !SKIP 1 !MAXIT 500

wt ~ mu age !r animal

but asreml tell me Singularity in Average Information Matrix, I know from
the asreml manual that I need possibly modify model (overspecified) ,but
the model is very simple. How can I get the heritability of body weight ?
thanks.

Luan sheng