diff --git a/Dummies.UnitTests/AnyCollectionTests.cs b/Dummies.UnitTests/AnyCollectionTests.cs
index 6fac877..c62d724 100644
--- a/Dummies.UnitTests/AnyCollectionTests.cs
+++ b/Dummies.UnitTests/AnyCollectionTests.cs
@@ -351,6 +351,19 @@ public void DictionaryContainingKeyInsideDomainDoesNotInflate() {
         Check.That(conflict.Message).Contains("2 distinct value");
     }
 
+    [Fact(DisplayName = "ContainingAnyKey: a key drawn from a generator is forced into the dictionary; null is rejected (issue #287).")]
+    public void DictionaryContainingAnyKeyForcesADrawnKey() {
+        // The drawn key (4242) lies outside the key generator's own {1..9} domain, so it is supplied directly and
+        // extends the effective cardinality — the ContainingAny path, now reaching dictionary keys.
+        for (int i = 0; i < SampleCount; i++) {
+            Dictionary<int, string> dictionary =
+                Any.DictionaryOf(Any.Int32().Between(1, 9), Any.String().NonEmpty()).NonEmpty().ContainingAnyKey(Any.Int32().OneOf(4242)).Generate();
+            Check.That(dictionary.ContainsKey(4242)).IsTrue();
+        }
+
+        Check.ThatCode(() => Any.DictionaryOf(Any.Int32(), Any.Int32()).ContainingAnyKey(null!)).Throws<ArgumentNullException>();
+    }
+
     [Fact(DisplayName = "PairOf and TripleOf assemble value tuples from constrained parts.")]
     public void PairAndTriple() {
         for (int i = 0; i < SampleCount; i++) {
diff --git a/Dummies/AnyDictionary.cs b/Dummies/AnyDictionary.cs
index 38933ae..4009c6c 100644
--- a/Dummies/AnyDictionary.cs
+++ b/Dummies/AnyDictionary.cs
@@ -95,6 +95,23 @@ public AnyDictionary<TKey, TValue> ContainingKey(TKey key) {
         return With(_keys.WithContaining(key, $"ContainingKey({AnyDerivation.Display(key)})"));
     }
 
+    /// <summary>
+    ///     Requires the dictionary to contain an entry whose key is drawn from <paramref name="generator" /> at
+    ///     generation time — the key analogue of a collection's <c>ContainingAny</c>. Named apart from
+    ///     <see cref="ContainingKey" /> to keep the two cases legible: <see cref="ContainingKey" /> pins a concrete
+    ///     key known now, whereas this draws one from a generator when the dictionary is built. The drawn key takes
+    ///     one entry's room and its value is generated like any other.
+    /// </summary>
+    /// <param name="generator">The generator whose drawn key the dictionary must contain.</param>
+    /// <returns>A new generator carrying the added constraint.</returns>
+    /// <exception cref="ArgumentNullException">Thrown when <paramref name="generator" /> is <c>null</c>.</exception>
+    /// <exception cref="ConflictingAnyConstraintException">Thrown when the constraint contradicts a constraint already declared.</exception>
+    public AnyDictionary<TKey, TValue> ContainingAnyKey(IAny<TKey> generator) {
+        if (generator is null) { throw new ArgumentNullException(nameof(generator)); }
+
+        return With(_keys.WithContaining(generator, "ContainingAnyKey(<generator>)"));
+    }
+
     /// <inheritdoc />
     public Dictionary<TKey, TValue> Generate() {
         List<TKey>                 keys       = _keys.Materialize(_source ?? AmbientRandomSource.Instance);
diff --git a/Dummies/PublicAPI/net8.0/PublicAPI.Unshipped.txt b/Dummies/PublicAPI/net8.0/PublicAPI.Unshipped.txt
index 913325c..078d195 100644
--- a/Dummies/PublicAPI/net8.0/PublicAPI.Unshipped.txt
+++ b/Dummies/PublicAPI/net8.0/PublicAPI.Unshipped.txt
@@ -116,6 +116,7 @@ Dummies.AnyDecimal.OneOf(params decimal[]! values) -> Dummies.AnyDecimal!
 Dummies.AnyDecimal.Positive() -> Dummies.AnyDecimal!
 Dummies.AnyDecimal.Zero() -> Dummies.AnyDecimal!
 Dummies.AnyDictionary<TKey, TValue>
+Dummies.AnyDictionary<TKey, TValue>.ContainingAnyKey(Dummies.IAny<TKey>! generator) -> Dummies.AnyDictionary<TKey, TValue>!
 Dummies.AnyDictionary<TKey, TValue>.ContainingKey(TKey key) -> Dummies.AnyDictionary<TKey, TValue>!
 Dummies.AnyDictionary<TKey, TValue>.Empty() -> Dummies.AnyDictionary<TKey, TValue>!
 Dummies.AnyDictionary<TKey, TValue>.Generate() -> System.Collections.Generic.Dictionary<TKey, TValue>!
diff --git a/Dummies/PublicAPI/netstandard2.0/PublicAPI.Unshipped.txt b/Dummies/PublicAPI/netstandard2.0/PublicAPI.Unshipped.txt
index 5bc982e..de183a7 100644
--- a/Dummies/PublicAPI/netstandard2.0/PublicAPI.Unshipped.txt
+++ b/Dummies/PublicAPI/netstandard2.0/PublicAPI.Unshipped.txt
@@ -101,6 +101,7 @@ Dummies.AnyDecimal.OneOf(params decimal[]! values) -> Dummies.AnyDecimal!
 Dummies.AnyDecimal.Positive() -> Dummies.AnyDecimal!
 Dummies.AnyDecimal.Zero() -> Dummies.AnyDecimal!
 Dummies.AnyDictionary<TKey, TValue>
+Dummies.AnyDictionary<TKey, TValue>.ContainingAnyKey(Dummies.IAny<TKey>! generator) -> Dummies.AnyDictionary<TKey, TValue>!
 Dummies.AnyDictionary<TKey, TValue>.ContainingKey(TKey key) -> Dummies.AnyDictionary<TKey, TValue>!
 Dummies.AnyDictionary<TKey, TValue>.Empty() -> Dummies.AnyDictionary<TKey, TValue>!
 Dummies.AnyDictionary<TKey, TValue>.Generate() -> System.Collections.Generic.Dictionary<TKey, TValue>!