stem doesn't fit in beam

[Jean-marc LEGRAND]

Hi list !

A weird message, and I can't figure out what it means (it worked very well till 
this afternoon !).
Here's my script :

tenor =     \notes
            \relative c' {
            \override Staff.TimeSignature  #'style = #'default
            \override Rest #'style = #'neo_mensural
            \time 4/4
            \key d \minor
                \clef "petrucci_c3"
r1
r
r
r
r
r4 a' a bes8. a16
g4 c4. bes8 [a8. g16]
fis2 f4. g8
e4 f4. [f8 f8. f16]
g4. g8 f [f f8. f16]
f8 [d e fis] g4. g8
g4. g8 f4 g8. f16
e4 c f4. ees8
d4 d8 ees d4. d8
d4. d8 e4 f8. ees16
d4 ees f4. f8
f [d e fis] g4. g8
fis4. fis!8 g4 g8 f
ees8 [f ees! d] c d ees!4
d4. d8 d2
\bar "|."
}



And here's the message :

warning: stem doesn't fit in beam:
e4       f4. [f8 f8. f16]


Everything seems OK, and I've already used such beaming. What's wrong with the 
stems ?