Why does Typharm Limited's Docusate Sodium Adult 50 mg/5 ml Oral Solution carry a warning for people with phenylketonuria?

Typharm Limited's Docusate Sodium Adult 50 mg/5 ml Oral Solution contains 15 mg aspartame per 5 ml dose, a source of phenylalanine that may harm people with phenylketonuria because their bodies cannot remove phenylalanine properly, causing it to build up.