diff --git a/lib/ControlSystemsBase/src/types/LFTSystem.jl b/lib/ControlSystemsBase/src/types/LFTSystem.jl
index 7c3a737f8..1975261f3 100644
--- a/lib/ControlSystemsBase/src/types/LFTSystem.jl
+++ b/lib/ControlSystemsBase/src/types/LFTSystem.jl
@@ -114,15 +114,15 @@ end
 
 function append(systems::LFTT...) where LFTT <: LFTSystem
     timeevol = common_timeevol(systems...)
-    A   = blockdiag(s.A for s in systems)
-    B1  = blockdiag(s.B1 for s in systems)
-    B2  = blockdiag(s.B2 for s in systems)
-    C1  = blockdiag(s.C1 for s in systems)
-    C2  = blockdiag(s.C2 for s in systems)
-    D11 = blockdiag(s.D11 for s in systems)
-    D12 = blockdiag(s.D12 for s in systems)
-    D21 = blockdiag(s.D21 for s in systems)
-    D22 = blockdiag(s.D22 for s in systems)
+    A   = blockdiag(s.P.A for s in systems)
+    B1  = blockdiag(s.P.B1 for s in systems)
+    B2  = blockdiag(s.P.B2 for s in systems)
+    C1  = blockdiag(s.P.C1 for s in systems)
+    C2  = blockdiag(s.P.C2 for s in systems)
+    D11 = blockdiag(s.P.D11 for s in systems)
+    D12 = blockdiag(s.P.D12 for s in systems)
+    D21 = blockdiag(s.P.D21 for s in systems)
+    D22 = blockdiag(s.P.D22 for s in systems)
     LFTT(
         PartitionedStateSpace(A,B1,B2,C1,C2,D11,D12,D21,D22,timeevol),
         reduce(vcat, [feedback_channel(s) for s in systems])
diff --git a/lib/ControlSystemsBase/test/test_delayed_systems.jl b/lib/ControlSystemsBase/test/test_delayed_systems.jl
index 6828e95fb..b8038d47a 100644
--- a/lib/ControlSystemsBase/test/test_delayed_systems.jl
+++ b/lib/ControlSystemsBase/test/test_delayed_systems.jl
@@ -56,6 +56,22 @@ P2_fr = (im*ω .+ 1) ./ (im*ω .+ 2)
 @test freqresp(P2 * delay(1), ω)[:] ≈ P2_fr .* exp.(-im*ω) rtol=1e-15
 @test freqresp(delay(1) * P2, ω)[:] ≈ P2_fr .* exp.(-im*ω) rtol=1e-15
 
+## append
+P12 = append(P1, P2)
+G12 = [P1 tf(0); tf(0) P2]
+
+F = freqresp(P12, ω)
+
+@test freqrespv(P1, ω) ≈ F[1,1,:]
+@test freqrespv(P2, ω) ≈ F[2,2,:]
+@test all(iszero, F[1,2,:])
+@test all(iszero, F[2,1,:])
+
+F = freqresp(G12, ω)
+@test freqrespv(P1, ω) ≈ F[1,1,:]
+@test freqrespv(P2, ω) ≈ F[2,2,:]
+@test all(iszero, F[1,2,:])
+@test all(iszero, F[2,1,:])
 
 # Equality
 @test P1 == deepcopy(P1)
diff --git a/lib/ControlSystemsBase/test/test_hammerstein_wiener.jl b/lib/ControlSystemsBase/test/test_hammerstein_wiener.jl
index fdacefa3e..035e74056 100644
--- a/lib/ControlSystemsBase/test/test_hammerstein_wiener.jl
+++ b/lib/ControlSystemsBase/test/test_hammerstein_wiener.jl
@@ -1,6 +1,7 @@
 using ControlSystemsBase
 using ControlSystemsBase: HammersteinWienerSystem
 using SparseArrays
+using Test
 
 
 # @testset "test_nonlinearity_system" begin