--- a/test/SbpOperators/volumeops/derivatives/second_derivative_test.jl	Wed Feb 25 14:33:42 2026 +0100
+++ b/test/SbpOperators/volumeops/derivatives/second_derivative_test.jl	Mon Mar 02 15:58:27 2026 +0100
@@ -21,12 +21,18 @@
     @testset "Constructors" begin
         @testset "1D" begin
             Dₓₓ = second_derivative(g_1D, stencil_set)
+            @test Dₓₓ == second_derivative(g_1D, stencil_set, 1)
             @test Dₓₓ == second_derivative(g_1D, inner_stencil, closure_stencils)
             @test Dₓₓ isa LazyTensor{Float64,1,1}
+
+            @test_throws DomainError(3, "Derivative direction must be 1.") second_derivative(g_1D, stencil_set, 3)
         end
         @testset "2D" begin
             Dₓₓ = second_derivative(g_2D,stencil_set,1)
             @test Dₓₓ isa LazyTensor{Float64,2,2}
+
+
+            @test_throws DomainError(3, "Derivative direction must be in 1:2.") second_derivative(g_2D, stencil_set, 3)
         end
     end