Find the probability of the following event :

$\displaystyle P(A \cap B | A \cup B ) $

Attempt to solution :

$\displaystyle P(A \cap B | A \cup B )= \frac{P(A \cap B \cap A \cup B)}{P(A \cup B)}= \frac{P(A)}{P(A \cup B)} $

I don't understand how l am getting a different answer from this one ? Can someone please help me