There are (at least) two ways of reading this. The first is probably how you are reading it: the assumption caused an error. In this case, it is probably not

There are (at least) two ways of reading this. The first is probably how you are reading it: the assumption caused an error. In this case, it is probably not a "probably," since an error did indeed occur. A second way to read it is whether it was an error on their part to make the assumption, and that is harder to say since they could have just been following the data they had on hand (a "you couldn't have known" scenario).