warn of (someone or something)
1. To provide information in advance about some potential source of harm, danger, or trouble. Analysts have been warning of a likely downturn in the economy for months now, so this dip in the market shouldn't come as a surprise. Police are warning of a number of escaped inmates who are at large in the area.
2. To caution someone about something; to inform someone of the risks or dangers of something. In this usage, a noun or pronoun is used between "warn" and "of." My mom always warns me of the dangers of texting while driving. I wish someone has warned me of her kleptomania before we started dating.