Posting customer invoice issue.

We are able to create a Customer Master Record, but when trying to post the customer invoice, then an error msg comes up saying

Customer 75 is not defined in company code 5959

I know very well that I did define it and defined many others but I still keep on getting this error. Also when I try to display the customer master record it says it has not been created. This is very strange because I did create it but why am i getting this error.

Please help on this.