diff --git a/src/Extents.jl b/src/Extents.jl
index b630378..be43f76 100644
--- a/src/Extents.jl
+++ b/src/Extents.jl
@@ -123,21 +123,26 @@ for all dimensions.
 
 $ORDER_DOC
 """
+union(ext1::Extent{()}, ext2::Extent{()}; strict=false) = Extent()
 function union(ext1::Extent, ext2::Extent; strict=false)
     _maybe_check_keys_match(ext1, ext2, strict) || return nothing
-    keys = _shared_keys(ext1, ext2)
-    if length(keys) == 0
-        return nothing
-    else
-        values = map(keys) do k
-            k = _unwrap(k)
-            k_exts = (ext1[k], ext2[k])
-            a = min(map(first, k_exts)...)
-            b = max(map(last, k_exts)...)
-            (a, b)
+    keys = _all_keys(ext1, ext2)
+    values = map(keys) do k
+        k = _unwrap(k)
+        if haskey(ext1, k)
+            if haskey(ext2, k)
+                k_exts = (ext1[k], ext2[k])
+                a = min(map(first, k_exts)...)
+                b = max(map(last, k_exts)...)
+                (a, b)
+            else
+                ext1[k]
+            end
+        else
+            ext2[k]
         end
-        return Extent{map(_unwrap, keys)}(values)
     end
+    return Extent{map(_unwrap, keys)}(values)
 end
 union(a::Extent, ::Nothing; strict=false) = strict ? nothing : a
 union(::Nothing, b::Extent; strict=false) = strict ? nothing : b
@@ -501,6 +506,12 @@ function _shared_keys(ext1::Extent{K1}, ext2::Extent{K2}) where {K1,K2}
         k in K2 ? (acc..., Val{k}()) : acc
     end
 end
+# Use NamedTuple merge for combining keys
+function _all_keys(ext1::Extent{K1}, ext2::Extent{K2}) where {K1,K2}
+    ntk1 = NamedTuple{K1}(map(k -> Val{k}(), K1)) 
+    ntk2 = NamedTuple{K2}(map(k -> Val{k}(), K2))
+    return values(merge(ntk1, ntk2))
+end
 
 @noinline _ext_no_key(key) = throw(ErrorException("Extent has no field $key"))
 
diff --git a/test/runtests.jl b/test/runtests.jl
index fed423a..e4affbd 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -57,9 +57,10 @@ end
     a = E(X=(0.1, 0.5), Y=(1.0, 2.0))
     b = E(X=(2.1, 2.5), Y=(3.0, 4.0), Z=(0.0, 1.0))
     c = E(Z=(0.2, 2.0))
-    @test Extents.union(a, b) == Extents.union(a, b, a) == E(X=(0.1, 2.5), Y=(1.0, 4.0))
+    @test Extents.union(a, b) == Extents.union(a, b, a) == E(X=(0.1, 2.5), Y=(1.0, 4.0), Z=(0.0, 1.0))
     @test Extents.union(a, b; strict=true) === nothing
-    @test Extents.union(a, c) === nothing
+    @test Extents.union(a, c) == E(X=(0.1, 0.5), Y=(1.0, 2.0), Z=(0.2, 2.0))
+    @test Extents.union(a, b) == E(X=(0.1, 2.5), Y=(1.0, 4.0), Z=(0.0, 1.0))
 
     # If either argument is nothing, return the other
     @test Extents.union(a, nothing) === a
@@ -69,7 +70,7 @@ end
     # If both arguments are nothing, return nothing
     @test Extents.union(nothing, nothing) === nothing
     @test Extents.union(nothing, nothing; strict=true) === nothing
-end
+# end
 
 @testset "covers" begin
     # An extent contains itself