Cannot export facial animations via fbx

Hi there, im currently working on a game in unity which involves character animation, however when it comes to exporting the facial animations for the character blender does not like it and comes up with this error message

Whenever I export the other animations such as walking etc it seems to work fine and what makes it weirder is that i had made a rig before for another project the same way as this one with no problems with the facial animation what so ever, any ideas?

i export via FBX 7.4 binary with selected objects checked aswell as armature and mesh selected.

thanks - invader

Are you using shape keys for the expression?