diff --git a/LEXICON_STYLE_GUIDE.md b/LEXICON_STYLE_GUIDE.md
index bbb5ca51..d446f855 100644
--- a/LEXICON_STYLE_GUIDE.md
+++ b/LEXICON_STYLE_GUIDE.md
@@ -52,7 +52,7 @@ This document summarizes the ATProto Lexicon Style Guide rules that are checked
 - **Primitives**: Use appropriate primitive types (string, integer, boolean)
 - **Formats**: Apply format constraints (datetime, uri, cid, did, etc.)
 - **Refs**: Use refs for reusable types and relationships
-- **Unions**: Use unions when a property can be one of several types
+- **Unions**: Use unions when a property can be one of several types. All variants in a union must be object or record types (strings/numbers are not allowed directly in unions by ATProto spec).
 
 #### Constraints