Fix test error

Author: YingboMa

test/mass_matrix.jl

@@ -9,7 +9,7 @@ eqs = [D(y[1]) ~ -k[1]*y[1] + k[3]*y[2]*y[3],
        0 ~  y[1] + y[2] + y[3] - 1]
 
 sys = ODESystem(eqs,t,y,k)
-@test_throws ArgumentError ODESystem(eqs,y)
+@test_throws ArgumentError ODESystem(eqs,y[1])
 M = calculate_massmatrix(sys)
 @test M == [1 0 0
             0 1 0