Fix wrong way of calling differentiator

include/differential.h

@@ -17,7 +17,7 @@ class ScalarDifferential
 
     Vector<valtype> operator()(Coordinate<argtype> x) const override {
         assert(x.dimension() == input_dimension());
-        return diff(func, x, Vector<valtype>(x.dimension(), h));
+        return diff.diff(func, x, Vector<valtype>(x.dimension(), h));
     }
 
     size_t input_dimension() const override { return func.input_dimension(); }