Official Description
6.7.3 It is an error if the result of an activation of a function is undefined upon completion of the algorithm of the activation.
Simplified Description
In other words, it is an error if the result of a function call is undefined.
Error Handling
This error is reported if checking for undefined values is enabled and the type of the function's result is one of the supported types (see Run-Time Project Options in the Irie Pascal User's Manual).