Pattern Matching Map

hey,
So I have this function
def update(
conn,
%{
“expense_id” => id,
“name” => _name,
ocr => _ocr

    } = payload
  ) do ...

my ocr field is a map that I’m going to update :
when I compiled my code I had : cannot use variable ocr as map key inside a pattern. Map keys in patterns can only be literals (such as atoms, strings, tuples, etc.) or an existing variable matched with the pin operator (such as ^some_var) as error
Do you have any suggestions? did any one get by this problem?