Indesit WEI 147 Error Code H20 on Rinse?

My Indesit Washing machine comes up with the Error Code H20. It only does this when it tries rinse, using the Fabric Conditioner drawer compartment.

It puts a very small amount of water in the compartment (not enough to activate the siphonic action) and flashes the error code.

Otherwise the machine fills, washes etc... If I select Rinse and Spin, it fills the machine for rinse using the other drawer compartments fine?

What are the likely causes and how do I fix it?