nnir problem with mailboxes with umlauts (oh sorry, the last mail was HTML)

[postmaster]

It seems that nnir has problems with mailboxes which have umlauts in
there names.

For my search tests I created these two mailboxes on my dovecot IMAP-server:
"dick" and "dünn".
When I tried to search a string in these boxes (with G G) I got this
"result" for the mailbox "dünn":
gnus-select-newsgroup: Couldn’t request group nnir:nnir-riek1dwxvv5.fsf: Search 
produced empty results.
(The same search in the mailbox "dick" works fine.)

I switched on the debug-on-entry for the function nnir-run-imap and got
this result (at the mailbox "dünn"):

Debugger entered--entering a function:
* nnir-run-imap(((query . "dick")) "nnimap:buckthorn's IMAP server" 
("d\303\274nn"))
#f(compiled-function (x) #<bytecode 0x1aaf37d>)(("nnimap:buckthorn's IMAP 
server" ("d\303\274nn")))
mapcar(#f(compiled-function (x) #<bytecode 0x1aaf37d>) (("nnimap:buckthorn's 
IMAP server" ("d\303\274nn"))))
nnir-run-query(((nnir-query-spec (query . "dick")) (nnir-group-spec 
("nnimap:buckthorn's IMAP server" ("d\303\274nn")))))
nnir-request-group("nnir-rieftokxud4.fsf" "nnir-ephemeral" t 
("nnir:nnir-rieftokxud4.fsf" 3 nil nil (nnir "nnir-ephemeral" (nnir-address 
"nnir")) ((quit-config #<killed buffer> . group) (nnir-specs (nnir-query-spec 
(query . "dick")) (nnir-group-spec ("nnimap:buckthorn's IMAP server" 
("d\303\274nn")))) (nnir-artlist))))
gnus-request-group("nnir:nnir-rieftokxud4.fsf" t nil 
("nnir:nnir-rieftokxud4.fsf" 3 nil nil (nnir "nnir-ephemeral" (nnir-address 
"nnir")) ((quit-config #<killed buffer> . group) (nnir-specs (nnir-query-spec 
(query . "dick")) (nnir-group-spec ("nnimap:buckthorn's IMAP server" 
("d\303\274nn")))) (nnir-artlist))))
gnus-select-newsgroup("nnir:nnir-rieftokxud4.fsf" t nil)
gnus-summary-read-group-1("nnir:nnir-rieftokxud4.fsf" t t nil nil nil)
gnus-summary-read-group("nnir:nnir-rieftokxud4.fsf" t t nil nil nil nil)
gnus-group-read-group(t t "nnir:nnir-rieftokxud4.fsf" nil)
gnus-group-read-ephemeral-group("nnir-rieftokxud4.fsf" (nnir "nnir") nil nil 
nil nil ((nnir-specs (nnir-query-spec (query . "dick")) (nnir-group-spec 
("nnimap:buckthorn's IMAP server" ("d\303\274nn")))) (nnir-artlist)))
gnus-group-make-nnir-group(nil)
funcall-interactively(gnus-group-make-nnir-group nil)
call-interactively(gnus-group-make-nnir-group nil nil)
command-execute(gnus-group-make-nnir-group)

For me it seems that this is a problem in nnir because doveadm has no
problems with these searches:
doveadm search mailbox dick text "dick"
18b74815af68fb5c540900001b5d164c 1
doveadm search mailbox dünn text "dick"
18b74815af68fb5c540900001b5d164c 1

Am I right that this is a bug?