Void controls implicit in algorithms

I read algorithms in a CS book in which a potentially null value is passed to a function, but neither the caller nor the called party checks whether the value is zero. Is this a common practice in algorithms?

To be more precise, I implements a red-black tree and the grandparent of a node is transmitted to a rotation function, but if he correctly understands the algorithm, the large -parent may be zero at the time of the call of the rotation function.

I should probably turn such calls into a conditional block, but I was curious to know if algorithms are often specified this way and if there are additional tips on how to handle these cases.

Thank you