Here is code that provoked this error (though the error message as been made more specific in later releases):

ghost function Eval(): string -> bool {

ghost function EvalOperator(op: string -> bool): string -> bool 
  (v: string) => op(v)

function Dummy(str: string): bool
  requires str == []

The problem has to do with arrow types. In particular here, the argument of EvalOperator takes a -> function, which is a total, heap-independent function. However, its argument, Dummy, is a partial, heap-independent function, because it has a precondition. If you want EvalOperator to be flexible enough to take partial functions, then declare op to have the type string --> bool.

There is more on arrow types in the reference manual;