Invalid request hash, please check request parameters and how you generate the hash.

Hi, I'm getting this error, and I'm not sure what I'm doing wrong.

The documentation says "hash: hashed with SHA-512 algorithm of the appended params containing(referenceNumber + amount + currency + payer.phoneNumber + payer.email + payee.accountNumber + payee.phoneNumber + payee.bankId + payee.bankAccountNumber + hashkey)"

I generate the string to hash in exactly this order without the + and (). I then use SHA-512 to hash then I pass it in the header this way:
'hash' : 'hashed string'