matchend() and \zs

My file - first line:
an 91.010 &System.-sep010- <Nop>

I want to copy all numbers after -sep
echo match(getline(1), 'sep\zs\d\+\ze-')
gives 23 as output.
All ok.

but
echo matchend(getline(1), 'sep\zs\d\+\ze-', 23)
gives -1 as output.
Why?

If I remove "sep" before '\zs' it works.
(echo matchend(getline(1), '\zs\d\+\ze-', 23)
gives 26 as output.

Does matchend not work with \zs?
How can I resolve this problem?

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