Integration with variable limits

Hi,

I am trying to solve a problem which requires that I define a function in the form:

V=-Integral of E(x) dx from x=0 until x=x
the function V is used in the coefficients of the equations which I am trying to solve.

I have tried the following approach:
V=-integral((E)*(dest(x)>=0)*(dest(x)<=(x)))
where integral is a model coupling integration operator. But It gives the following error:

"
Failed to find a solution for the initial parameter.
Singular matrix.

There are 62 void equations (empty rows in matrix) for the variable mod1.E.
at coordinates: (8.3326e-009), (9.76543e-009), (1.12679e-008), (1.28434e-008), (1.44956e-008), ...
Returned solution is not converged.
"
Anybody knows how to do it?
I need it very much.

Thanks on advance,

Nelson