diff --git a/src/constraint_matrix.jl b/src/constraint_matrix.jl
index 1e95aa1..997a037 100644
--- a/src/constraint_matrix.jl
+++ b/src/constraint_matrix.jl
@@ -83,26 +83,29 @@ Base.@propagate_inbounds function Base.getindex(
     return pos - neg
 end
 
-struct NZPairsIter{T}
-    m::ConstraintMatrix{T}
+struct NZPairsIter{T,I}
+    m::ConstraintMatrix{T,I}
 end
 
 Base.eltype(::Type{NZPairsIter{T}}) where {T} = Pair{Int,T}
 Base.IteratorSize(::Type{<:NZPairsIter}) = Base.SizeUnknown()
 
 # TODO: iterate over (idx=>val) pairs combining vals
-function Base.iterate(itr::NZPairsIter, state::Tuple{Int,Int} = (1, 1))
+function Base.iterate(
+    itr::NZPairsIter,
+    state::Tuple{Int,Nothing} = (1, nothing),
+)
     k = iterate(itr.m.pos, state[1])
-    isnothing(k) && return iterate(itr, state[2])
+    isnothing(k) && return iterate(itr, (nothing, 1))
     idx, st = k
-    return idx => itr.m.val, (st, 1)
+    return idx => itr.m.val, (st, nothing)
 end
 
-function Base.iterate(itr::NZPairsIter, state::Int)
-    k = iterate(itr.m.neg, state[1])
+function Base.iterate(itr::NZPairsIter, state::Tuple{Nothing,Int})
+    k = iterate(itr.m.neg, state[2])
     isnothing(k) && return nothing
     idx, st = k
-    return idx => -itr.m.val, st
+    return idx => -itr.m.val, (nothing, st)
 end
 
 """