Can't add new line in customize value field

[Lennart Borgman]

While looking at mmm-mode I came across the following problem (using CVS 
Emacs from today on w32):

Start with emacs -Q, evaluate the following defcustom and then do M-x 
customize-option RET. Now go to the value field (widget) and try to 
insert a new line by hitting RET. For me this moves the point to the 
next custom widget ("State"). C-h c RET in the value field gives 
`widget-field-activate'. What is wrong here?

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(defcustom mmm-major-mode-preferences
 '((perl cperl-mode perl-mode)
   (javascript javascript-mode c++-mode)
   (java jde-mode java-mode c++-mode)
   (css css-mode c++-mode))
 "User preferences about what major modes to use.
Each element has the form \(LANGUAGE . MODES) where LANGUAGE is the
name of a programming language such as `perl' as a symbol, and MODES
is a list of possible major modes to use, such as `cperl-mode' or
`perl-mode'.  The first element of MODES which is `fboundp' is used
for submodes of LANGUAGE.  The last element of MODES should be a mode
which will always be available."
 :group 'mmm
 :type '(repeat (cons symbol
                      (repeat
                       (restricted-sexp :match-alternatives
                                        (fboundp))))))
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