diff --git a/ruby/ext/google/protobuf_c/ruby-upb.h b/ruby/ext/google/protobuf_c/ruby-upb.h
index 512a822..34b8689 100755
--- a/ruby/ext/google/protobuf_c/ruby-upb.h
+++ b/ruby/ext/google/protobuf_c/ruby-upb.h
@@ -3382,6 +3382,10 @@
   return upb_Message_GetField(msg, field, def).str_val;
 }
 
+// Sets the value of a `string` or `bytes` field. The bytes of the value are not
+// copied, so it is the caller's responsibility to ensure that they remain valid
+// for the lifetime of `msg`. That might be done by copying them into the given
+// arena, or by fusing that arena with the arena the bytes live in, for example.
 UPB_API_INLINE bool upb_Message_SetString(upb_Message* msg,
                                           const upb_MiniTableField* field,
                                           upb_StringView value, upb_Arena* a) {