\smallCaps how to

[yota moteuchi]

\version "2.11.20"

Given this piece of the titling scheme

scoreTitleMarkup = \markup {
    \column {
        \on-the-fly #print-all-headers { \bookTitleMarkup \hspace #1 }
        \line {
            \fromproperty #'header:piece 
        }
    }
}

I would like to display the piece name as small caps.
\smallCaps "string" seems to work but when I put it in front of a complex _expression_ like \fromproperty #'header:piece I got a bunch of errors

Preprocessing graphical objects.../home/yoochan/usr/lilypond/usr/bin/../share/lilypond/current/scm/define-markup-commands.scm:980:64: In procedure string->list in _expression_ (string->list text):
/home/yoochan/usr/lilypond/usr/bin/../share/lilypond/current/scm/define-
markup-commands.scm:980:64: Wrong type argument in position 1 (expecting string): (#<procedure fromproperty-markup (layout props symbol)> header:piece)

And once again, I'm not fluent enough in scheme to catch the subtilities.

Thank you for your help

Yota