fix issue 130

src/LazyOperators/lazy_operator.jl

@@ -287,6 +287,10 @@ get_operator(a::NullOperator) = nothing
 get_args(a::NullOperator) = (nothing,)
 materialize(a::NullOperator, x) = a
 
+Base.length(::NullOperator) = 1
+Base.iterate(a::NullOperator) = (a, nothing)
+Base.iterate(a::NullOperator, state) = nothing
+
 function show_lazy_operator(op::NullOperator; level = 1, indent = 4, islast = (true,))
     level == 1 && println("\n---------------")
     print_tree_prefix(level, indent, islast)

src/assembler/assembler.jl

@@ -287,10 +287,13 @@ function _append_contribution!(X, I, J, U, V, values, elementInfo::CellInfo, dom
     idofs = get_dofs(V, icell, nV) # lines correspond to the TestFunction
     _idofs, _jdofs = _cartesian_product(idofs, jdofs)
     unwrapValues = _unwrap_cell_integrate(V, values)
-    append!(I, _idofs)
-    append!(J, _jdofs)
-    for _v in unwrapValues
-        append!(X, _v)
+    val = [reduce((x, y) -> (x..., y...), unwrapValues)...]
+    for (_i, _j, _v) in zip(_idofs, _jdofs, val)
+        if !isa(_v, NullOperator)
+            append!(X, _v)
+            append!(I, _i)
+            append!(J, _j)
+        end
     end
     return nothing
 end

test/runtests.jl

@@ -163,4 +163,8 @@ fname2sum = Dict(r[2] => r[1] for r in eachrow(f))
     custom_include("./operator/test_algebra.jl")
     custom_include("./dof/test_meshdata.jl")
     custom_include("./writers/test_vtk.jl")
+
+    @testset "Issues" begin
+        custom_include("./issues/issue_130.jl")
+    end
 end