diff --git a/lib/Context/RootContext.php b/lib/Context/RootContext.php
index 950a867..3b938e8 100644
--- a/lib/Context/RootContext.php
+++ b/lib/Context/RootContext.php
@@ -6,7 +6,7 @@
 declare(strict_types=1);
 namespace JKingWeb\Arsse\Context;
 
-class RootContext extends AbstractContext {
+abstract class RootContext extends AbstractContext {
     public $limit = 0;
     public $offset = 0;
 
diff --git a/lib/Context/UnionContext.php b/lib/Context/UnionContext.php
index 257e501..db5a98f 100644
--- a/lib/Context/UnionContext.php
+++ b/lib/Context/UnionContext.php
@@ -10,7 +10,7 @@ class UnionContext extends RootContext implements \ArrayAccess, \Countable, \Ite
     protected $contexts = [];
 
     public function offsetExists(mixed $offset): bool {
-        return array_key_exists($offset, $this->contexts);
+        return isset($this->contexts[$offset]);
     }
 
     public function offsetGet(mixed $offset): mixed {
@@ -18,7 +18,12 @@ class UnionContext extends RootContext implements \ArrayAccess, \Countable, \Ite
     }
 
     public function offsetSet(mixed $offset, mixed $value): void {
-        $this->contexts[$offset ?? count($this->contexts)] = $value;
+        assert($value instanceof RootContext, new \Exception("Union contexts may only contain other non-exclusion contexts"));
+        if (isset($offset)) {
+            $this->contexts[$offset] = $value;
+        } else {
+            $this->contexts[] = $value;
+        }
     }
 
     public function offsetUnset(mixed $offset): void {
@@ -35,7 +40,7 @@ class UnionContext extends RootContext implements \ArrayAccess, \Countable, \Ite
         }
     }
 
-    public function __construct(Context ...$context) {
+    public function __construct(RootContext ...$context) {
         $this->contexts = $context;
     }
 }
diff --git a/tests/cases/Misc/TestContext.php b/tests/cases/Misc/TestContext.php
index 1f8b638..f02f6fa 100644
--- a/tests/cases/Misc/TestContext.php
+++ b/tests/cases/Misc/TestContext.php
@@ -8,12 +8,14 @@ namespace JKingWeb\Arsse\TestCase\Misc;
 
 use JKingWeb\Arsse\Context\Context;
 use JKingWeb\Arsse\Context\ExclusionContext;
+use JKingWeb\Arsse\Context\UnionContext;
 use JKingWeb\Arsse\Misc\Date;
 use JKingWeb\Arsse\Misc\ValueInfo;
 
 /**
  * @covers \JKingWeb\Arsse\Context\Context<extended>
  * @covers \JKingWeb\Arsse\Context\ExclusionContext<extended>
+ * @covers \JKingWeb\Arsse\Context\UnionContext<extended>
  */
 class TestContext extends \JKingWeb\Arsse\Test\AbstractTest {
     protected $ranges = ['modifiedRange', 'markedRange', 'articleRange', 'editionRange'];
@@ -150,4 +152,27 @@ class TestContext extends \JKingWeb\Arsse\Test\AbstractTest {
         $this->assertSame($c1, $c1->not->article(null));
         $this->assertSame($c2, $c2->not->article(null));
     }
+
+    public function testExerciseAUnionContext(): void {
+        $c1 = new UnionContext;
+        $c2 = new Context;
+        $c3 = new UnionContext;
+        $this->assertSame(0, sizeof($c1));
+        $c1[] = $c2;
+        $c1[2] = $c3;
+        $this->assertSame(2, sizeof($c1));
+        $this->assertSame($c2, $c1[0]);
+        $this->assertSame($c3, $c1[2]);
+        $this->assertSame(null, $c1[1]);
+        unset($c1[0]);
+        $this->assertFalse(isset($c1[0]));
+        $this->assertTrue(isset($c1[2]));
+        $c1[] = $c2;
+        $act = [];
+        foreach($c1 as $k => $v) {
+            $act[$k] = $v;
+        }
+        $exp = [2 => $c3, $c2];
+        $this->assertSame($exp, $act);
+    }
 }