uso di \overrightarrow con \textbf

Buonasera,
quando utilizzo la seguente combinazione di comandi, al fine di ottenere un simbolo in grassetto con la freccia sopra:
`\overrightarrow{\textbf{u}}`
pdflatex non mi da problemi, mentre se cerco di fare la medesima cosa con una lettera dell’alfabeto greco, ad esempio:
`\overrightarrow{\textbf{\omega}}`
mi da come errore:
! Missing $ inserted.

$
l.3 …+ \int_A \overrightarrow{\textbf{ \omega }}
I’ve inserted a begin-math/end-math symbol since I think
you left one out. Proceed, with fingers crossed.
! Extra }, or forgotten $.
\textdef@ …th {#1}\let \f@size #2\selectfont #3}
}
l.3 …+ \int_A \overrightarrow{\textbf{ \omega }}
I’ve deleted a group-closing symbol because it seems to be
spurious, as in `$x}$’. But perhaps the } is legitimate and
you forgot something else, as in `\hbox{$x}’. In such cases
the way to recover is to insert both the forgotten and the
deleted material, e.g., by typing `I$}’.
! Extra }, or forgotten $.
\textdef@ …h {#1}\let \f@size #2\selectfont #3}}
l.3 …+ \int_A \overrightarrow{\textbf{ \omega }}
I’ve deleted a group-closing symbol because it seems to be
spurious, as in `$x}$’. But perhaps the } is legitimate and
you forgot something else, as in `\hbox{$x}’. In such cases
the way to recover is to insert both the forgotten and the
deleted material, e.g., by typing `I$}’.
! Extra }, or forgotten $.
\text@ …e {\textdef@ \displaystyle \f@size {#1}}
{\textdef@ \textstyle \f@s…
l.3 …+ \int_A \overrightarrow{\textbf{ \omega }}
I’ve deleted a group-closing symbol because it seems to be
spurious, as in `$x}$’. But perhaps the } is legitimate and
you forgot something else, as in `\hbox{$x}’. In such cases
the way to recover is to insert both the forgotten and the
deleted material, e.g., by typing `I$}’.

Dove sbaglio?
Grazie in anticipo