Error, (in dsolve) found the following equations not depending on the unknowns of the input system: ...

Hello there,

When I was trying to solve a differential equation with dsolve, I've got such a error report:

> dsolve({Q(0) = F[Az], diff(Q(x), x) = q[2](x)});

Error, (in dsolve) found the following equations not depending on the unknowns
of the input system: {Q(0) = -(1/4)*l^2*q[0]-(1/4)*l^2*q[0]*exp(2)+F}

But if I only write:

> dsolve(diff(Q(x),x)=q[2](x));

then there is no problem. 

Why I've got such a problem and how can I fix it? Thank you!