diff --git a/src/BitStringAddresses/bosefs.jl b/src/BitStringAddresses/bosefs.jl
index 8f7bfe410..8ac94c04e 100644
--- a/src/BitStringAddresses/bosefs.jl
+++ b/src/BitStringAddresses/bosefs.jl
@@ -24,7 +24,7 @@ automatically based on the properties of the address.
   particles in `bs` is equal to `N`.
 
 * [`@fs_str`](@ref): Addresses are sometimes printed in a compact manner. This
-  representation can also be used as a constructor. See the last example below.
+  representation can also be used as a constructor. See the examples below.
 
 # Examples
 
@@ -32,7 +32,7 @@ automatically based on the properties of the address.
 julia> BoseFS{6,5}(0, 1, 2, 3, 0)
 BoseFS{6,5}(0, 1, 2, 3, 0)
 
-julia> BoseFS([abs(i - 3) ≤ 1 ? i - 1 : 0 for i in 1:5])
+julia> BoseFS(abs(i - 3) ≤ 1 ? i - 1 : 0 for i in 1:5)
 BoseFS{6,5}(0, 1, 2, 3, 0)
 
 julia> BoseFS(5, 2 => 1, 3 => 2, 4 => 3)
@@ -97,7 +97,8 @@ function BoseFS{N,M}(onr::Union{AbstractArray{<:Integer},NTuple{M,<:Integer}}) w
     end
     return BoseFS{N,M,S}(from_bose_onr(S, onr))
 end
-function BoseFS(onr::Union{AbstractArray,Tuple})
+function BoseFS(onr) # single argument constructor
+    onr = Tuple(onr)
     M = length(onr)
     N = sum(onr)
     return BoseFS{N,M}(onr)
@@ -106,10 +107,13 @@ BoseFS(vals::Integer...) = BoseFS(vals) # specify occupation numbers
 BoseFS(val::Integer) = BoseFS((val,)) # single mode address
 BoseFS{N,M}(vals::Integer...) where {N,M} = BoseFS{N,M}(vals)
 
+# Sparse constructors
 BoseFS(M::Integer, pairs::Pair...) = BoseFS(M, pairs)
 BoseFS(M::Integer, pairs) = BoseFS(sparse_to_onr(M, pairs))
 BoseFS{N,M}(pairs::Pair...) where {N,M} = BoseFS{N,M}(pairs)
 BoseFS{N,M}(pairs) where {N,M} = BoseFS{N,M}(sparse_to_onr(M, pairs))
+BoseFS(pairs::Pair...) = throw(ArgumentError("number of modes must be provided"))
+
 
 function print_address(io::IO, b::BoseFS{N,M}; compact=false) where {N,M}
     if compact && b.bs isa SortedParticleList
@@ -259,7 +263,7 @@ end
 
 Compute the new address of a hopping event for the Hubbard model. Returns the new
 address and the square root of product of occupation numbers of the involved modes
-multiplied by a term consistent with boundary condition as the `value`. 
+multiplied by a term consistent with boundary condition as the `value`.
 The following boundary conditions are supported:
 
 * `:periodic`: hopping over the boundary gives does not change the `value`.
diff --git a/src/BitStringAddresses/fermifs.jl b/src/BitStringAddresses/fermifs.jl
index 43f9dc3a6..94a548292 100644
--- a/src/BitStringAddresses/fermifs.jl
+++ b/src/BitStringAddresses/fermifs.jl
@@ -24,7 +24,7 @@ chosen automatically based on the properties of the address.
   particles in `bs` is equal to `N`, or whether each mode only contains one particle.
 
 * [`@fs_str`](@ref): Addresses are sometimes printed in a compact manner. This
-  representation can also be used as a constructor. See the last example below.
+  representation can also be used as a constructor. See the examples below.
 
 # Examples
 
@@ -95,7 +95,8 @@ function FermiFS{N,M}(onr::Union{AbstractArray{<:Integer},NTuple{M,<:Integer}})
     end
     return FermiFS{N,M,S}(from_fermi_onr(S, onr))
 end
-function FermiFS(onr::Union{AbstractArray,Tuple})
+function FermiFS(onr)
+    onr = Tuple(onr)
     M = length(onr)
     N = sum(onr)
     return FermiFS{N,M}(onr)
@@ -109,6 +110,7 @@ FermiFS(M::Integer, pairs::Pair...) = FermiFS(M, pairs)
 FermiFS(M::Integer, pairs) = FermiFS(sparse_to_onr(M, pairs))
 FermiFS{N,M}(pairs::Vararg{Pair,N}) where {N,M} = FermiFS{N,M}(pairs)
 FermiFS{N,M}(pairs) where {N,M} = FermiFS{N,M}(sparse_to_onr(M, pairs))
+FermiFS(pairs::Pair...) = throw(ArgumentError("number of modes must be provided"))
 
 function print_address(io::IO, f::FermiFS{N,M}; compact=false) where {N,M}
     if compact && f.bs isa SortedParticleList
diff --git a/src/BitStringAddresses/occupationnumberfs.jl b/src/BitStringAddresses/occupationnumberfs.jl
index 5383c3300..33a2349de 100644
--- a/src/BitStringAddresses/occupationnumberfs.jl
+++ b/src/BitStringAddresses/occupationnumberfs.jl
@@ -2,16 +2,19 @@
     OccupationNumberFS{M,T} <: SingleComponentFockAddress
 Address type that stores the occupation numbers of a single component bosonic Fock state
 with `M` modes. The occupation numbers must fit into the type `T <: Unsigned`. The number of
-particles is runtime data, and can be retrieved with `num_particles(address)`.
+particles is runtime data, and can be retrieved with [`num_particles(address)`](@ref).
 
 # Constructors
 - `OccupationNumberFS(val::Integer...)`: Construct from occupation numbers. Must be
   < 256 to fit into `UInt8`.
+- `OccupationNumberFS(M, pairs::Pair...)`: Construct from a sparse representation with
+  `M` modes and pairs of mode index and occupation number.
 - `OccupationNumberFS{[M,T]}(onr)`: Construct from collection `onr` with `M` occupation
-  numbers with type `T`. If unspecified, the type `T` of the occupation numbers is inferred
-  from the type of the arguments.
+  numbers with optional type `T`.  `onr` may be a tuple, an array, or a generator.
+- `OccupationNumberFS{M[,T]}()`: Construct a vacuum state with `M` modes. If `T` is
+  unspecified, `UInt8` is used.
 - `OccupationNumberFS(fs::BoseFS)`: Construct from [`BoseFS`](@ref).
-- With shortform macro [`@fs_str`](@ref). Specify the number of
+- With short form macro [`@fs_str`](@ref). Specify the number of
   significant bits in braces. See example below.
 
 # Examples
@@ -24,36 +27,78 @@ true
 
 julia> num_particles(ofs)
 6
+
+julia> OccupationNumberFS{5}() # vacuum state with 5 modes
+OccupationNumberFS{5, UInt8}(0, 0, 0, 0, 0)
+
+julia> OccupationNumberFS(i for i in 1:3) # use list comprehension
+OccupationNumberFS{3, UInt8}(1, 2, 3)
+
+julia> OccupationNumberFS(4, 1=>2, 3=>4) # sparse constructor
+OccupationNumberFS{4, UInt8}(2, 0, 4, 0)
+
+julia> OccupationNumberFS(5, i=>i^2 for i in 2:4) # sparse with list comprehension
+OccupationNumberFS{5, UInt8}(0, 4, 9, 16, 0)
 ```
 """
 struct OccupationNumberFS{M,T<:Unsigned} <: SingleComponentFockAddress{missing,M}
     onr::SVector{M,T}
+
+    function OccupationNumberFS{M,T}(args...) where {M,T<:Unsigned}
+        return new(SVector{M,T}(args...))
+    end
 end
 
-function OccupationNumberFS{M,T}(args...) where {M,T}
-    return OccupationNumberFS(SVector{M,T}(args...))
+function OccupationNumberFS(sv::SVector{M,T}) where {M,T<:Unsigned}
+    return OccupationNumberFS{M,T}(sv)
 end
 
-function OccupationNumberFS(args...)
-    sv = SVector(args...)
-    all(isinteger, sv) || throw(ArgumentError("all arguments must be integers"))
-    all(x -> x ≥ 0, sv) || throw(ArgumentError("all arguments must be non-negative"))
-    all(x -> x < 256, sv) || throw(ArgumentError("arguments don't fit in a byte, specify type"))
-    return OccupationNumberFS(SVector{length(sv),UInt8}(args...))
+function OccupationNumberFS(arg)
+    t = Tuple(arg)
+    return OccupationNumberFS{length(t)}(t)
+end
+
+function OccupationNumberFS(args::Number...)
+    return OccupationNumberFS(SVector(args))
+end
+OccupationNumberFS(arg::Number) = OccupationNumberFS{1}(tuple(arg)) # to resolve ambiguity
+function OccupationNumberFS{M}(args::Number...) where {M}
+    return OccupationNumberFS{M}(SVector{M}(args))
 end
 
 function OccupationNumberFS{M}(args...) where M
-    sv = SVector{M}(args...)
+    t = Tuple(args...)
+    if all(x -> isa(x, Pair), t)
+        return OccupationNumberFS{M}(t)
+    end
+    sv = SVector{M}(t)
     all(isinteger, sv) || throw(ArgumentError("all arguments must be integers"))
     all(x -> x ≥ 0, sv) || throw(ArgumentError("all arguments must be non-negative"))
     all(x -> x < 256, sv) || throw(ArgumentError("arguments don't fit in a byte, specify type"))
-    return OccupationNumberFS(SVector{M,UInt8}(args...))
+    return OccupationNumberFS{M,UInt8}(args...)
 end
 
 function OccupationNumberFS(fs::BoseFS{N,M}) where {N,M}
     return OccupationNumberFS{M,select_int_type(N)}(onr(fs))
 end
 
+# convenience constructors for vacuum state
+function OccupationNumberFS{M,T}() where {M,T<:Unsigned}
+    return OccupationNumberFS(SVector{M,T}(zero(T) for _ in 1:M))
+end
+OccupationNumberFS{M}() where {M} = OccupationNumberFS{M,UInt8}()
+
+# Sparse constructors
+OccupationNumberFS(M::Number, pairs::Pair...) = OccupationNumberFS(Int(M), pairs)
+OccupationNumberFS(M::Number, pairs) = OccupationNumberFS(sparse_to_onr(Int(M), pairs))
+OccupationNumberFS{M}(pairs::Pair...) where {M} = OccupationNumberFS{M}(pairs)
+
+function OccupationNumberFS{M}(pairs::NTuple{<:Any,Pair}) where {M}
+    OccupationNumberFS{M}(sparse_to_onr(M, pairs))
+end
+
+OccupationNumberFS(pairs::Pair...) = throw(ArgumentError("number of modes must be provided"))
+
 function print_address(io::IO, ofs::OccupationNumberFS{M,T}; compact=false) where {M,T}
     if compact
         BITS = sizeof(T) * 8
@@ -68,13 +113,13 @@ Base.reverse(ofs::OccupationNumberFS) = OccupationNumberFS(reverse(ofs.onr))
 onr(ofs::OccupationNumberFS) = ofs.onr
 function Base.isless(a::OccupationNumberFS{M}, b::OccupationNumberFS{M}) where {M}
     # equivalent to `isless(reverse(a.onr), reverse(b.onr))`
+    # reversing the order here to make it consistent with BoseFS
     i = length(a.onr)
     while i > 1 && a.onr[i] == b.onr[i]
         i -= 1
     end
     return isless(a.onr[i], b.onr[i])
 end
-# reversing the order here to make it consistent with BoseFS
 Base.:(==)(a::OccupationNumberFS, b::OccupationNumberFS) = a.onr == b.onr
 Base.hash(ofs::OccupationNumberFS, h::UInt) = hash(ofs.onr, h)
 
diff --git a/src/BitStringAddresses/sortedparticlelist.jl b/src/BitStringAddresses/sortedparticlelist.jl
index 2e094dc0d..d5ac18551 100644
--- a/src/BitStringAddresses/sortedparticlelist.jl
+++ b/src/BitStringAddresses/sortedparticlelist.jl
@@ -1,16 +1,16 @@
 """
-    select_int_type(M)
+    select_int_type(n)
 
-Select unsigned integer type that can hold values up to `M`.
+Select unsigned integer type that can hold values up to `n`.
 """
-function select_int_type(M)
-    if M ≤ 0
-        throw(ArgumentError("`M` must be positive!"))
-    elseif M ≤ typemax(UInt8)
+function select_int_type(n)
+    if n < 0
+        throw(ArgumentError("`n` must be a non-negative integer!"))
+    elseif n ≤ typemax(UInt8)
         return UInt8
-    elseif M ≤ typemax(UInt16)
+    elseif n ≤ typemax(UInt16)
         return UInt16
-    elseif M ≤ typemax(UInt32)
+    elseif n ≤ typemax(UInt32)
         return UInt32
     else
         return UInt64
@@ -20,7 +20,7 @@ end
 """
     SortedParticleList{N,M,T<:Unsigned}
 
-Type for storing sparse fock states. Stores the mode number of each particle as an entry
+Type for storing sparse Fock states. Stores the mode number of each particle as an entry
 with only its mode stored. The entries are always kept sorted.
 
 Iterating over `SortedParticleList`s yields occupied modes as a tuple of occupation number,
diff --git a/test/BitStringAddresses.jl b/test/BitStringAddresses.jl
index efe35896d..666977baa 100644
--- a/test/BitStringAddresses.jl
+++ b/test/BitStringAddresses.jl
@@ -184,7 +184,7 @@ end
 
         @test_throws ArgumentError BoseFS(10, 11 => 1)
         @test_throws ArgumentError BoseFS(10, 10 => -1)
-        @test_throws MethodError BoseFS(10 => 1)
+        @test_throws ArgumentError BoseFS(10 => 1)
     end
     @testset "onr" begin
         middle_full_onr = onr(middle_full)
@@ -461,7 +461,9 @@ end
     end
 
     @testset "OccupationNumberFS with multiple arguments" begin
-        @test isa(OccupationNumberFS(1, 2, 3), OccupationNumberFS{3, UInt8})
+        @test OccupationNumberFS(i for i in 1:3) == OccupationNumberFS(1, 2, 3)
+        @test isa(OccupationNumberFS{3,UInt32}(i for i in 1:3), OccupationNumberFS{3,UInt32})
+        @test isa(OccupationNumberFS(1, 2, 3), OccupationNumberFS{3,UInt8})
         @test_throws ArgumentError OccupationNumberFS(1.1, 2, 3)
         @test_throws ArgumentError OccupationNumberFS(-1, 2, 3)
         @test_throws ArgumentError OccupationNumberFS(1, 2, 300)
@@ -479,6 +481,20 @@ end
         @test isa(OccupationNumberFS(fs), OccupationNumberFS{2, UInt8})
         fs = BoseFS(1, 333)
         @test isa(OccupationNumberFS(fs), OccupationNumberFS{2,UInt16})
+        fs = BoseFS(0, 0)
+        @test isa(OccupationNumberFS(fs), OccupationNumberFS{2,UInt8})
+    end
+
+    @testset "OccupationNumberFS with sparse constructor" begin
+        @test OccupationNumberFS(2, 2=>4) == OccupationNumberFS(0, 4)
+        @test OccupationNumberFS{2}(2 => 4) == OccupationNumberFS(2, 2 => 4)
+        @test OccupationNumberFS(5, i => i + 1 for i in 1:3) ==
+            OccupationNumberFS{5}(i => i + 1 for i in 1:3) ==
+            OccupationNumberFS{5}(Tuple(i => i + 1 for i in 1:3)) ==
+            OccupationNumberFS(5, 1 => 2, 2 => 3, 3 => 4) ==
+            OccupationNumberFS{5,UInt8}(2, 3, 4, 0, 0)
+        @test OccupationNumberFS{5}(i => i^2 for i in 1:5) ==
+            OccupationNumberFS(5, i => i^2 for i in 1:5)
     end
 
     @testset "Printing and parsing OccupationNumberFS" begin