Ḍaʿf al-Taʾlīf (ضعف التأليف) occurs when composition violates an established Arabic syntactic pattern and thereby impairs the expression.
The source gives: ضَرَبَ غُلَامُهُ زَيْدًا
His servant struck Zayd.
If the suffix pronoun in “his servant” is intended to refer to Zayd, its referent follows both in wording and in grammatical rank. The issue is not that the words are individually invalid, but that the pronoun–antecedent relation creates a syntactic defect.