Passing Record Type parameter to API. Field in the API call is null

I have a wrapper program that calls API with various parameters that include one record type too. As soon as the wrapper is run, the API returns error. Looks like the data passed through record type parameter is passing null values to API. What could be the possibilities that when an API is called with record type parameter, the values as seen by API are null? When I look for the variable passed to one of the columns within that record before calling API, it has value, but when API is called, it is null. Any pointers will be helpful