[ksh] Warning: pipe symbol within ${} should be quoted?

With syntax-check ('ksh -n') I get a warning in Ksh for this expression

  "${pipe#* | }"

concerning the pipe symbol. (Bash and Zsh don't complain.)

(2709)$ ksh -n -c '"${pipe#* | }"'
ksh: warning: line 1: | within ${} should be quoted
(2710)$ ksh -n -c '"${pipe#* [|] }"'
ksh: warning: line 1: | within ${} should be quoted
(2711)$ ksh -n -c '"${pipe#* \| }"'

(2712)$ bash -n -c '"${pipe#* | }"'
(2713)$ bash -n -c '"${pipe#* [|] }"'
(2714)$ bash -n -c '"${pipe#* \| }"'

(2715)$ zsh -n -c '"${pipe#* | }"'
(2716)$ zsh -n -c '"${pipe#* [|] }"'
(2717)$ zsh -n -c '"${pipe#* \| }"'

Just a bug, or is ksh too sensible here, or some potential subtle issue?

Janis