diff --git a/docs/csharp/language-reference/builtin-types/union.md b/docs/csharp/language-reference/builtin-types/union.md
index 760b376c8e80e..33d9288766d83 100644
--- a/docs/csharp/language-reference/builtin-types/union.md
+++ b/docs/csharp/language-reference/builtin-types/union.md
@@ -171,6 +171,9 @@ The compiler prefers `TryGetValue` over the `Value` property when implementing p
 
 ### Union member providers
 
+> [!NOTE]
+> If an `IUnionMembers` interface is used, all union members must be defined on that interface.
+
 A union type can delegate its union members to a nested `IUnionMembers` interface. When this interface is present, the compiler looks for `Create` factory methods instead of constructors:
 
 :::code language="csharp" source="snippets/unions/MemberProvider.cs" id="MemberProvider":::