Paypal gateway failing on version 1.8

[guidoferreyra]

I just updated merx from 1.7.x to 1.8.1 and when using Paypal gateway I get the following error:

The payment could not be initialized."
--
key | "error.merx.initializePayment"
file | ".../site/plugins/merx/src/Merx.php"
line | 256
details | Object { message: 'Undefined array key "id"', code: 2, file: ".../site/plugins/merx/src/Gateways.php", … }
message | 'Undefined array key "id"'
code | 2
file | ".../site/plugins/merx/src/Gateways.php"
line | 56
code | 500

Let me know how can I help you to debug this.