We propose a novel approach to stream definition and manipulation, which reconciles, in a sense, lazy evaluation and regular corecursion. Our solution is based on two key ideas. Corecursion is enhanced, by allowing in equations defining streams other typical operators besides the stream constructor; in this way, some non-regular streams are definable. Furthermore, execution includes a runtime check to ensure that the stream generated by a function call is well-defined, in the sense that access to an arbitrary index always succeeds. In this way we achieve a convenient trade-off between expressive power and reliability.