diff --git a/base/generator.jl b/base/generator.jl
index b0f7e32d0b22f..73d0e8a2e3ee2 100644
--- a/base/generator.jl
+++ b/base/generator.jl
@@ -126,3 +126,4 @@ IteratorEltype(::Type) = HasEltype()  # HasEltype is the default
 IteratorEltype(::Type{Generator{I,T}}) where {I,T} = EltypeUnknown()
 
 IteratorEltype(::Type{Any}) = EltypeUnknown()
+IteratorEltype(::Type{Union{}}) = EltypeUnknown()
diff --git a/base/iterators.jl b/base/iterators.jl
index 1564ffa1be9a1..ac3c3ad88d9a3 100644
--- a/base/iterators.jl
+++ b/base/iterators.jl
@@ -880,6 +880,7 @@ _flatteneltype(I, et) = EltypeUnknown()
 flatten_iteratorsize(::Union{HasShape, HasLength}, ::Type{<:NTuple{N,Any}}) where {N} = HasLength()
 flatten_iteratorsize(::Union{HasShape, HasLength}, ::Type{<:Tuple}) = SizeUnknown()
 flatten_iteratorsize(::Union{HasShape, HasLength}, ::Type{<:Number}) = HasLength()
+flatten_iteratorsize(::Union{HasShape, HasLength}, ::Type{Union{}}) = SizeUnknown()
 flatten_iteratorsize(a, b) = SizeUnknown()
 
 _flatten_iteratorsize(sz, ::EltypeUnknown, I) = SizeUnknown()
diff --git a/test/iterators.jl b/test/iterators.jl
index 5fca4f11b91ad..6e5f88a72cd0d 100644
--- a/test/iterators.jl
+++ b/test/iterators.jl
@@ -365,6 +365,7 @@ end
 @test length(flatten(zip(1:3, 4:6))) == 6
 @test length(flatten(1:6)) == 6
 @test collect(flatten(Any[])) == Any[]
+@test collect(flatten(())) == Union{}[]
 @test_throws ArgumentError length(flatten(NTuple[(1,), ()])) # #16680
 @test_throws ArgumentError length(flatten([[1], [1]]))