A DESCRIPTION OF THE PROBLEM :
This API https://docs.oracle.com/en/java/javase/14/docs/api/java.base/java/util/stream/IntStream.html#reduce(int,java.util.function.IntBinaryOperator) requires that the IntBinaryOperator is an associative accumulation function, as the documentation mentions.

Down below, the documentation mentions "Sum, min, max, and average [...]", then goes on to exemplify how to compute a sum.

Mentioning "average" also may imply to some readers that it can be computed using reduce(), however this is not the case as average is not an associative operation.