Add iszero check in linear equation detection

Author: YingboMa

src/systems/systemstructure.jl

@@ -194,7 +194,7 @@ function find_linear_equations(sys)
             c = expand_derivatives(Differential(var)(term), false)
             # test if `var` is linear in `eq`.
             if !(c isa Symbolic) && c isa Number
-                if isinteger(c)
+                if isinteger(c) && !iszero(c)
                     c = convert(Integer, c)
                     linear_term += c * var
                     push!(coeffs, c)

test/reduction.jl

@@ -191,3 +191,10 @@ reducedsol = solve(nlprob, NewtonRaphson())
 residual = fill(100.0, length(states(reducedsys)))
 nlprob.f(residual, reducedsol.u, pp)
 @test all(x->abs(x) < 1e-5, residual)
+
+N = 5
+@variables xs[1:N]
+A = reshape(1:N^2, N, N)
+eqs = xs .~ A * xs
+sys′ = NonlinearSystem(eqs, xs, [])
+sys = structural_simplify(sys′)