Why did I get API error code 81002?

We return this NVP/SOAP API error when we identify your provided method as “unspecified” due to detected coding errors.

Debug your code for possible solutions. Here's how:

Don't use urlencode + http_build_query because this will result in double encoding.
Check that you aren't using a semicolon (;) as a separator. We support use of only ampersands (&) as separators.
Review your code to ensure that none of the variables have been deprecated.