ImageMagick bug?

Subject: imagemagick: If the trailing character of a montage label option is a percent sign, the output image is erroneous
Package: imagemagick
Version: 6:6.2.3.6-3
Severity: normal

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Executing this command line:

montage -geometry +10+10 -tile 2 -label "-sepia-tone 15%%" \
ImageMagick_Chapter5_Insert20a.jpg -label "-sepia-tone 35%%" \
ImageMagick_Chapter5_Insert20b.jpg -label "-sepia-tone 55%%" \
ImageMagick_Chapter5_Insert20c.jpg -label "-sepia-tone 75%%" \
ImageMagick_Chapter5_Insert20d.jpg ImageMagick_Chapter5_Insert20.jpg
Whereas this command line:

montage -geometry +10+10 -tile 2 -label "-sepia-tone 15%% " \
ImageMagick_Chapter5_Insert20a.jpg -label "-sepia-tone 35%%" \
ImageMagick_Chapter5_Insert20b.jpg -label "-sepia-tone 55%%" \
ImageMagick_Chapter5_Insert20c.jpg -label "-sepia-tone 75%%" \
ImageMagick_Chapter5_Insert20d.jpg ImageMagick_Chapter5_Insert20.jpg

Produces the expected output:
-- System Information:
Debian Release: testing/unstable
  APT prefers unstable
    APT policy: (500, 'unstable'), (500, 'testing'), (500, 'stable')
    Architecture: i386 (i686)
    Shell:  /bin/sh linked to /bin/bash
    Kernel: Linux 2.6.11
    Locale: LANG=C, LC_CTYPE=C (charmap=ANSI_X3.4-1968)

    Versions of packages imagemagick depends on:
    ii  libmagick6                   6:6.2.3.6-3 Image manipulation library

    imagemagick recommends no packages.

    -- no debconf information