Oracle Message ORA-06573 - Function string modifies package state, cannot be used here

ERRORFunction string modifies package state, cannot be used here
CAUSEThere are two possible causes for this message: * A SQL statement references a packaged, PL/SQL function that does not contain a pragma containing the Write no Package State (WNPS). * A SQL statement references a stand-alone, PL/SQL function that modifies a package state. A stand-alone, PL/SQL function referenced by a SQL statement cannot modify a package state.
ACTIONIf the function is a packaged, PL/SQL function: Recreate the function and include a pragma containing the Write no Package State (WNPS). If the function is a stand-alone, PL/SQL function: Delete the function from the SQL statement.