Possible problem when comparing key with a replace() value

[naoliv]

Trying to validated the attached example file using this rule:

way[highway][ref][wikipedia][ref = replace(tag("wikipedia"), "pt:", "")] {
        throwError: tr("foo");
}

I don't see the expected error message.
Using println I can see that replace(tag("wikipedia"), "pt:", "") is giving the correct string (SP-225)

Am I missing something or there is some kind of problem here, please?

JOSM:

Repository Root: http://josm.openstreetmap.de/svn
Build-Date: 2015-10-15 01:32:10
Last Changed Author: Don-vip
Revision: 8882
Repository UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Relative URL: ^/trunk
URL: http://josm.openstreetmap.de/svn/trunk
Last Changed Date: 2015-10-15 01:02:36 +0200 (Thu, 15 Oct 2015)
Last Changed Rev: 8882

Identification: JOSM/1.5 (8882 pt_BR) Linux Debian GNU/Linux testing (stretch)
Memory Usage: 707 MB / 9102 MB (443 MB allocated, but free)
Java version: 1.7.0_85, Oracle Corporation, OpenJDK 64-Bit Server VM
Java package: openjdk-7-jre:amd64-7u85-2.6.1-3
VM arguments: [-Dawt.useSystemAAFontSettings=on]
Dataset consistency test: No problems found

Plugins:
- Create_grid_of_ways (31603)
- OpeningHoursEditor (31603)
- PicLayer (31603)
- RoadSigns (31603)
- SimplifyArea (31603)
- buildings_tools (31603)
- conflation (0.1.7)
- contourmerge (1013)
- download_along (31603)
- editgpx (31603)
- geotools (31603)
- imagery_offset_db (31603)
- importvec (31603)
- jts (31603)
- log4j (31603)
- measurement (31603)
- merge-overlap (31603)
- opendata (31241)
- pbf (31603)
- pdfimport (31603)
- poly (31603)
- reverter (31625)
- scripting (30710)
- todo (29154)
- turnrestrictions (31603)
- undelete (31603)
- utilsplugin2 (31603)

[simon04]

This selector works:

way[highway][ref][wikipedia][tag("ref") = replace(tag("wikipedia"), "pt:", "")]

[naoliv]

Hum... is it necessary to always use a tag() for cases like this?