diff --git a/doc/bird.sgml b/doc/bird.sgml
index 38f70bf9..087f76b7 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -3591,10 +3591,14 @@ some of them (marked with `<tt/O/') are optional.
 	name="local role"> is configured it set automatically.
 </descrip>
 
-<p>For attributes unknown by BIRD, the user can assign a name (on top level)
-to an attribute by its number. This defined name can be used then to both set
-(by a bytestring literal, transitive) or unset the given attribute even though
-BIRD knows nothing about it:
+<p>For attributes unknown by BIRD, the user can assign a name (on top level) to
+an attribute by its number. This defined name can be used then to get, set (as a
+bytestring, transitive) or unset the given attribute even though BIRD knows
+nothing about it.
+
+<p>Note that it is not possible to define an attribute with the same number
+as one known by BIRD, therefore use of this statement carries a risk of
+incompatibility with future BIRD versions.
 
 <tt><label id="bgp-attribute-custom">attribute bgp <m/number/ bytestring <m/name/;</tt>
 
diff --git a/filter/f-inst.c b/filter/f-inst.c
index 4aa4c489..dab6cdfb 100644
--- a/filter/f-inst.c
+++ b/filter/f-inst.c
@@ -893,6 +893,12 @@
     ARG_ANY(1);
     DYNAMIC_ATTR;
     ARG_TYPE(1, da->type);
+
+    FID_NEW_BODY;
+      if (da->type == T_OPAQUE)
+	cf_error("Setting opaque attribute is not allowed");
+
+    FID_INTERPRET_BODY;
     {
       struct eattr *a;
 
diff --git a/proto/bgp/bgp.h b/proto/bgp/bgp.h
index b59b02d8..b79fca29 100644
--- a/proto/bgp/bgp.h
+++ b/proto/bgp/bgp.h
@@ -615,6 +615,7 @@ void bgp_unset_attr(ea_list **to, uint code);
 
 int bgp_encode_mp_reach_mrt(struct bgp_write_state *s, eattr *a, byte *buf, uint size);
 
+const char * bgp_attr_name(uint code);
 int bgp_encode_attrs(struct bgp_write_state *s, ea_list *attrs, byte *buf, byte *end);
 ea_list * bgp_decode_attrs(struct bgp_parse_state *s, byte *data, uint len);
 void bgp_finish_attrs(struct bgp_parse_state *s, ea_list **to);