[j-nsp] Prepending customer AS results in AS23456

[Markus]

Hi list!

I'm trying to prepend a customers 4-byte ASN to their prefixes. JunOS 
12.3R2.5 on M7i. This is my config, simplified:

[routing-options static]
route a.a.a.a/22 {
     next-hop b.b.b.b;
     readvertise;
}

[policy-options]
policy-statement UPSTREAM-OUT {
     term 1 {
         from {
             route-filter a.a.a.a/22 exact;
         }
         then {
             as-path-expand 222222;
         }
     }

UPSTREAM-OUT is applied as export to my upstream's BGP peer. So far so good:

admin at router# run show route advertising-protocol bgp my-upstream-ip

Prefix           Nexthop           MED   Lclpref    AS path
* a.a.a.a/22     Self              10               222222 I

But when I check a public looking glass I can see that 222222 becomes 23456:

3257 1299 my-upstream my-asn 23456

I changed "as-path-expand" to "as-path-prepend" and this resulted in the 
BGP session to drop and I can see this in my log:

router rpd[1567]: bgp_read_v4_message:10289: NOTIFICATION received from 
my-upstream-ip (External AS my-upstream): code 3 (Update Message Error) 
subcode 11 (AS path attribute problem), Data:  40 02 0a 02 02 00

Does this maybe mean that my upstream routers software does not yet 
support 4-byte ASNs?

I'm also doing the same to various peers and there it works fine. I can 
see the path "my-asn 222222" in my peers looking glasses.

Thanks!
Markus