':' in variable text

[Michel Lemaitre]

I cannot put chord names containing ':' in variables.
For example 

toto = { la2:dim }

/Users/mich1/Music/LilyPond/lesFeuillesMortes.ly:68:14: 
Erreur : unrecognized string, not in text 
script or \lyricmode
toto = { la2:
             dim }

Note that 

toto =  { la2 }

works !