I think this is because of F#'s type inference, the error message certainly seems to suggest this:

"This error may be due to limitations associated with generic recursion, i.e. type inference requires that generic functions and members are instantiated in only one way within their own recursive scope, e.g. within a 'let rec' collection or within a group of classes. Use an independent 'let rec' binding to create a function which can be reused at multiple types."

What exactly are you trying to achieve?

Thanks,
Rob