"Error! Could not connect!" on test

Not sure whether this is a bug or configuration issue. On the config page, the "Test Stripe Connection" button gives the following error. In the log I get the error: "Warning: call_user_func() expects parameter 1 to be a valid callback, class 'Stripe\Account' not found in stripe_api_call() (line 31 of /.../sites/all/modules/stripe_api/stripe_api.methods.inc)."

Using latest API version on Stripe, Stripe PHP library 5.6.0. However, in the Status Report, I get the "Stripe PHP Library Missing" error. It's most definitely there, and Drupal Commerce has no issues accessing it. Hence I suspect a config issue - any ideas? Thanks.