diff --git a/src/backends/quantumoptics/quantumoptics.jl b/src/backends/quantumoptics/quantumoptics.jl
index f1b97284..e5c657c5 100644
--- a/src/backends/quantumoptics/quantumoptics.jl
+++ b/src/backends/quantumoptics/quantumoptics.jl
@@ -1,6 +1,6 @@
 import QuantumOpticsBase
 import QuantumOpticsBase: GenericBasis, CompositeBasis,
-    StateVector, AbstractSuperOperator, Ket, Operator,
+    StateVector, AbstractSuperOperator, Ket, Operator,basis,
     basisstate, spinup, spindown, sigmap, sigmax, sigmay, sigmaz, destroy,
     projector, identityoperator, embed, dm, expect, ptrace, spre, spost
 import QuantumOptics
@@ -37,7 +37,7 @@ function observable(state::Union{<:Ket,<:Operator}, indices, operation)
     expect(op, state)
 end
 
-function project_traceout!(state::Union{Ket,Operator},stateindex,psis::Base.AbstractVecOrTuple{Ket})
+function project_traceout!(state::Union{Ket,Operator},stateindex,psis::Base.AbstractVecOrTuple{})
     if nsubsystems(state) == 1 # TODO is there a way to do this in a single function, instead of _overlap vs _project_and_drop
         _overlaps = [_overlap(psi,state) for psi in psis]
         branch_probs = cumsum(_overlaps)
diff --git a/src/baseops/apply.jl b/src/baseops/apply.jl
index d2fb15f9..1228935e 100644
--- a/src/baseops/apply.jl
+++ b/src/baseops/apply.jl
@@ -18,11 +18,11 @@ function apply!(regs::Vector{Register}, indices::Vector{Int}, operation; time=no
     uptotime!(regs, indices, max_time)
     subsystemcompose(regs,indices)
     state = regs[1].staterefs[indices[1]].state[]
-    if !(basis(state)==basis(operation)) # e.g., performing a 2 qubit operation on a 4 qubit ket state
+    if (typeof(state) <: Ket) && !(basis(state)==basis(operation)) # e.g., performing a 2 qubit operation on a 4 qubit ket state
         state_indices_app = [r.stateindices[i] for (r,i) in zip(regs, indices)] # state indices on which we are performing the op
         state_indices = [idx for idx in 1:length(basis(state).bases) if idx ∉ state_indices_app] #state indices on which no op is performed
     else
-        state_indices = []
+        state_indices = [r.stateindices[i] for (r,i) in zip(regs, indices)]
     end
     state = apply!(state, state_indices, operation)
     regs[1].staterefs[indices[1]].state[] = state