diff --git a/src/ProtocolZoo/cutoff.jl b/src/ProtocolZoo/cutoff.jl
index 4d399475..f975f05a 100644
--- a/src/ProtocolZoo/cutoff.jl
+++ b/src/ProtocolZoo/cutoff.jl
@@ -69,7 +69,9 @@ end
 
             unlock(slot)
         end
-        if !isnothing(prot.period)
+        if isnothing(prot.period)
+            @yield onchange_tag(reg)
+        else
             @yield timeout(prot.sim, prot.period)
         end
     end
diff --git a/src/semaphore.jl b/src/semaphore.jl
index 31effde8..5f6df27f 100644
--- a/src/semaphore.jl
+++ b/src/semaphore.jl
@@ -1,6 +1,6 @@
 using ConcurrentSim
 using ResumableFunctions
-import Base: unlock, lock
+import Base: unlock, lock, islocked
 
 """Multiple processes can wait on this semaphore for a permission to run given by another process"""
 struct AsymmetricSemaphore
@@ -27,3 +27,7 @@ function unlock(s::AsymmetricSemaphore)
         unlock(s.lock)
     end
 end
+
+function islocked(s::AsymmetricSemaphore)
+    return islocked(s.lock)
+end
diff --git a/test/test_protocolzoo_cutoffprot.jl b/test/test_protocolzoo_cutoffprot.jl
index 5dc00dc6..0d4a965c 100644
--- a/test/test_protocolzoo_cutoffprot.jl
+++ b/test/test_protocolzoo_cutoffprot.jl
@@ -27,4 +27,36 @@ run(sim, 2.0)
 run(sim, 6.0)
 @test !isassigned(net[1][1])
 @test isassigned(net[2][1])
+
+# test for period=nothing
+net = RegisterNet([Register(2), Register(1)])
+sim = get_time_tracker(net)
+initialize!((net[1][1], net[2][1]), (Z1⊗Z1+Z2⊗Z2)/(sqrt(2.0)))
+tag!(net[1][1], EntanglementCounterpart, 2, 1)
+tag!(net[2][1], EntanglementCounterpart, 1, 1)
+
+cprot1 = CutoffProt(sim, net, 1; period=nothing, retention_time=3.0)
+cprot2 = CutoffProt(sim, net, 2; period=0.1, retention_time=3.0)
+@process cprot1()
+@process cprot2()
+
+@resumable function wait_and_tag(sim)
+    @yield timeout(sim, 5)
+    tag!(net[1][2], Int)
 end
+
+@process wait_and_tag(sim)
+
+run(sim, 2.0)
+@test isassigned(net[1][1])
+@test isassigned(net[2][1])
+
+run(sim, 4.0)
+@test isassigned(net[1][1])
+@test !isassigned(net[2][1])
+
+run(sim, 6.0)
+@test !isassigned(net[1][1])
+@test !isassigned(net[1][1])
+
+end
\ No newline at end of file