fix NLSys test

Author: anandijain

src/systems/diffeqs/odesystem.jl

@@ -377,8 +377,10 @@ function convert_system(::Type{<:ODESystem}, sys, t; name=nameof(sys))
         end
     end
     sub = Base.Fix2(substitute, varmap)
-    iv = only(independent_variables(sys))
-    sub.x[iv] = t # otherwise the Differentials aren't fixed
+    if sys isa AbstractODESystem
+        iv = only(independent_variables(sys))
+        sub.x[iv] = t # otherwise the Differentials aren't fixed
+    end
     neweqs = map(sub, equations(sys))
     defs = Dict(sub(k) => sub(v) for (k, v) in defaults(sys))
     return ODESystem(neweqs, t, newsts, parameters(sys); defaults=defs, name=name, checks=false)