# Summation error

I have:

``````s1 = x[i, 3]*x[j, l]
``````

But if I do

``````s1a = Sum[s1, {k, 4}] // Expand,
``````

even though k is not part of, it’s performing summation and giving wrong answer as:
$$-4 x_{3,i} x_{j,l}$$
How to avoid this error?
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