Double subscript caused error during compilation

Double subscript caused error during compilation

I have in code:
$\varphi(x) = \sum_{k=1}^ra_k\mathds{1}_{B_k}(x) $
but pdflatex commmand generates often:
?
! Undefined control sequence.
l.1354 ...rc \xi)(\omega) = \sum_{k=1}^ra_k\mathds
{1}_{B_k}(\xi(\omega))
I don't know how to repair this error. It often displays because I use
\mathds{1} for indicator function. How to fix it ?
Edit:
\mathds{} comes from \usepackage{dsfont}