diff --git a/src/libraries/System.Collections.Immutable/ref/System.Collections.Immutable.net9.cs b/src/libraries/System.Collections.Immutable/ref/System.Collections.Immutable.net9.cs
index 4cde549aac366..9a22864bd7e24 100644
--- a/src/libraries/System.Collections.Immutable/ref/System.Collections.Immutable.net9.cs
+++ b/src/libraries/System.Collections.Immutable/ref/System.Collections.Immutable.net9.cs
@@ -22,9 +22,9 @@ public abstract partial class FrozenDictionary<TKey, TValue>
     }
     public abstract partial class FrozenSet<T>
     {
-        public System.Collections.Frozen.FrozenSet<T>.AlternateLookup<TAlternate> GetAlternateLookup<TAlternate>() { throw null; }
-        public bool TryGetAlternateLookup<TAlternate>(out System.Collections.Frozen.FrozenSet<T>.AlternateLookup<TAlternate> lookup) { throw null; }
-        public readonly partial struct AlternateLookup<TAlternate>
+        public System.Collections.Frozen.FrozenSet<T>.AlternateLookup<TAlternate> GetAlternateLookup<TAlternate>() where TAlternate : allows ref struct { throw null; }
+        public bool TryGetAlternateLookup<TAlternate>(out System.Collections.Frozen.FrozenSet<T>.AlternateLookup<TAlternate> lookup) where TAlternate : allows ref struct { throw null; }
+        public readonly partial struct AlternateLookup<TAlternate> where TAlternate : allows ref struct
         {
             private readonly object _dummy;
             private readonly int _dummyPrimitive;