diff --git a/BitFaster.Caching/CacheDebugView.cs b/BitFaster.Caching/CacheDebugView.cs
index e7f02e3a..f418ea1f 100644
--- a/BitFaster.Caching/CacheDebugView.cs
+++ b/BitFaster.Caching/CacheDebugView.cs
@@ -6,6 +6,7 @@ namespace BitFaster.Caching
 {
     [ExcludeFromCodeCoverage]
     internal class CacheDebugView<K, V>
+        where K : notnull
     {
         private readonly ICache<K, V> cache;
 
diff --git a/BitFaster.Caching/Lfu/Builder/LfuBuilderBase.cs b/BitFaster.Caching/Lfu/Builder/LfuBuilderBase.cs
index c89386ac..af6967e2 100644
--- a/BitFaster.Caching/Lfu/Builder/LfuBuilderBase.cs
+++ b/BitFaster.Caching/Lfu/Builder/LfuBuilderBase.cs
@@ -12,6 +12,7 @@ namespace BitFaster.Caching.Lfu.Builder
     /// <typeparam name="TBuilder">The type of the builder.</typeparam>
     /// <typeparam name="TCacheReturn">The return type of the builder.</typeparam>
     public abstract class LfuBuilderBase<K, V, TBuilder, TCacheReturn> where TBuilder : LfuBuilderBase<K, V, TBuilder, TCacheReturn>
+        where K : notnull
     {
         internal readonly LfuInfo<K> info;
 
diff --git a/BitFaster.Caching/Lfu/Builder/LfuInfo.cs b/BitFaster.Caching/Lfu/Builder/LfuInfo.cs
index 963daacf..a7d06400 100644
--- a/BitFaster.Caching/Lfu/Builder/LfuInfo.cs
+++ b/BitFaster.Caching/Lfu/Builder/LfuInfo.cs
@@ -7,6 +7,7 @@
 namespace BitFaster.Caching.Lfu.Builder
 {
     internal sealed class LfuInfo<K>
+        where K : notnull
     {
         private object? expiry = null;
 
diff --git a/BitFaster.Caching/Lfu/LfuNode.cs b/BitFaster.Caching/Lfu/LfuNode.cs
index ed80f61b..5d37ad5e 100644
--- a/BitFaster.Caching/Lfu/LfuNode.cs
+++ b/BitFaster.Caching/Lfu/LfuNode.cs
@@ -2,6 +2,7 @@
 namespace BitFaster.Caching.Lfu
 {
     internal class LfuNode<K, V>
+        where K : notnull
     {
         internal LfuNodeList<K, V> list;
         internal LfuNode<K, V> next;
@@ -66,6 +67,7 @@ internal enum Position
     }
 
     internal sealed class AccessOrderNode<K, V> : LfuNode<K, V>
+        where K : notnull
     {
         public AccessOrderNode(K k, V v) : base(k, v)
         {
diff --git a/BitFaster.Caching/Lru/AfterAccessPolicy.cs b/BitFaster.Caching/Lru/AfterAccessPolicy.cs
index 04dcbf24..98c6c5dd 100644
--- a/BitFaster.Caching/Lru/AfterAccessPolicy.cs
+++ b/BitFaster.Caching/Lru/AfterAccessPolicy.cs
@@ -7,6 +7,7 @@ namespace BitFaster.Caching.Lru
     /// Implement an expire after access policy.
     /// </summary>
     internal readonly struct AfterAccessPolicy<K, V> : IItemPolicy<K, V, LongTickCountLruItem<K, V>>
+        where K : notnull
     {
         private readonly Duration timeToLive;
         private readonly Time time;
diff --git a/BitFaster.Caching/Lru/Builder/LruBuilderBase.cs b/BitFaster.Caching/Lru/Builder/LruBuilderBase.cs
index ab1eb208..e5a03b63 100644
--- a/BitFaster.Caching/Lru/Builder/LruBuilderBase.cs
+++ b/BitFaster.Caching/Lru/Builder/LruBuilderBase.cs
@@ -7,6 +7,7 @@ namespace BitFaster.Caching.Lru.Builder
     /// Recursive generic base class enables builder inheritance.
     /// </summary>
     public abstract class LruBuilderBase<K, V, TBuilder, TCacheReturn> where TBuilder : LruBuilderBase<K, V, TBuilder, TCacheReturn>
+        where K : notnull
     {
         internal readonly LruInfo<K> info;
 
diff --git a/BitFaster.Caching/Lru/Builder/LruInfo.cs b/BitFaster.Caching/Lru/Builder/LruInfo.cs
index 1e9355cc..ba69b5da 100644
--- a/BitFaster.Caching/Lru/Builder/LruInfo.cs
+++ b/BitFaster.Caching/Lru/Builder/LruInfo.cs
@@ -9,6 +9,7 @@ namespace BitFaster.Caching.Lru.Builder
     /// <typeparam name="K">The LRU key type</typeparam>
     // backcompat: make class internal
     public sealed class LruInfo<K>
+        where K : notnull
     {
         private object? expiry = null;
 
diff --git a/BitFaster.Caching/Lru/DiscretePolicy.cs b/BitFaster.Caching/Lru/DiscretePolicy.cs
index 01dc2c61..8e70a327 100644
--- a/BitFaster.Caching/Lru/DiscretePolicy.cs
+++ b/BitFaster.Caching/Lru/DiscretePolicy.cs
@@ -4,6 +4,7 @@
 namespace BitFaster.Caching.Lru
 {
     internal readonly struct DiscretePolicy<K, V> : IDiscreteItemPolicy<K, V>
+        where K : notnull
     {
         private readonly IExpiryCalculator<K, V> expiry;
         private readonly Time time;
diff --git a/BitFaster.Caching/Lru/IDiscreteItemPolicy.cs b/BitFaster.Caching/Lru/IDiscreteItemPolicy.cs
index ab6b432b..ec02d395 100644
--- a/BitFaster.Caching/Lru/IDiscreteItemPolicy.cs
+++ b/BitFaster.Caching/Lru/IDiscreteItemPolicy.cs
@@ -6,6 +6,7 @@
     /// <typeparam name="K"></typeparam>
     /// <typeparam name="V"></typeparam>
     public interface IDiscreteItemPolicy<K, V> : IItemPolicy<K, V, LongTickCountLruItem<K, V>>
+        where K : notnull
     {
     }
 }
diff --git a/BitFaster.Caching/Lru/IItemPolicy.cs b/BitFaster.Caching/Lru/IItemPolicy.cs
index af6ebdb6..8f3b588c 100644
--- a/BitFaster.Caching/Lru/IItemPolicy.cs
+++ b/BitFaster.Caching/Lru/IItemPolicy.cs
@@ -9,6 +9,7 @@ namespace BitFaster.Caching.Lru
     /// <typeparam name="V">The type of the value.</typeparam>
     /// <typeparam name="I">The type of the LRU item.</typeparam>
     public interface IItemPolicy<in K, in V, I> where I : LruItem<K, V>
+        where K : notnull
     {
         /// <summary>
         /// Creates an LRU item.
diff --git a/BitFaster.Caching/Lru/ITelemetryPolicy.cs b/BitFaster.Caching/Lru/ITelemetryPolicy.cs
index 9ca7501f..27d991cf 100644
--- a/BitFaster.Caching/Lru/ITelemetryPolicy.cs
+++ b/BitFaster.Caching/Lru/ITelemetryPolicy.cs
@@ -7,6 +7,7 @@ namespace BitFaster.Caching.Lru
     /// <typeparam name="K">The type of the key.</typeparam>
     /// <typeparam name="V">The type of the value.</typeparam>
     public interface ITelemetryPolicy<K, V> : ICacheMetrics, ICacheEvents<K, V>
+        where K : notnull
     {
         /// <summary>
         /// Increment the miss counter.
diff --git a/BitFaster.Caching/Lru/LongTickCountLruItem.cs b/BitFaster.Caching/Lru/LongTickCountLruItem.cs
index 31de3feb..3c218351 100644
--- a/BitFaster.Caching/Lru/LongTickCountLruItem.cs
+++ b/BitFaster.Caching/Lru/LongTickCountLruItem.cs
@@ -7,6 +7,7 @@ namespace BitFaster.Caching.Lru
     /// <typeparam name="K">The type of the key.</typeparam>
     /// <typeparam name="V">The type of the value.</typeparam>
     public class LongTickCountLruItem<K, V> : LruItem<K, V>
+        where K : notnull
     {
         /// <summary>
         /// Initializes a new instance of the LongTickCountLruItem class with the specified key and value.
diff --git a/BitFaster.Caching/Lru/LruItem.cs b/BitFaster.Caching/Lru/LruItem.cs
index 56944316..44e48939 100644
--- a/BitFaster.Caching/Lru/LruItem.cs
+++ b/BitFaster.Caching/Lru/LruItem.cs
@@ -7,6 +7,7 @@ namespace BitFaster.Caching.Lru
     /// <typeparam name="K">The type of the key.</typeparam>
     /// <typeparam name="V">The type of the value.</typeparam>
     public class LruItem<K, V>
+        where K : notnull
     {
         private volatile bool wasAccessed;
         private volatile bool wasRemoved;
diff --git a/BitFaster.Caching/Lru/LruPolicy.cs b/BitFaster.Caching/Lru/LruPolicy.cs
index 5982b6b9..cc1d93a0 100644
--- a/BitFaster.Caching/Lru/LruPolicy.cs
+++ b/BitFaster.Caching/Lru/LruPolicy.cs
@@ -7,6 +7,7 @@ namespace BitFaster.Caching.Lru
     /// Discards the least recently used items first. 
     /// </summary>
     public readonly struct LruPolicy<K, V> : IItemPolicy<K, V, LruItem<K, V>>
+        where K : notnull
     {
         ///<inheritdoc/>
         public TimeSpan TimeToLive => Defaults.Infinite;
diff --git a/BitFaster.Caching/Lru/NoTelemetryPolicy.cs b/BitFaster.Caching/Lru/NoTelemetryPolicy.cs
index b5aaf952..7afd78e5 100644
--- a/BitFaster.Caching/Lru/NoTelemetryPolicy.cs
+++ b/BitFaster.Caching/Lru/NoTelemetryPolicy.cs
@@ -10,6 +10,7 @@ namespace BitFaster.Caching.Lru
     /// <typeparam name="K">The type of the key.</typeparam>
     /// <typeparam name="V">The type of the value.</typeparam>
     public struct NoTelemetryPolicy<K, V> : ITelemetryPolicy<K, V>
+        where K : notnull
     {
         ///<inheritdoc/>
         public double HitRatio => 0.0;
diff --git a/BitFaster.Caching/Lru/TLruLongTicksPolicy.cs b/BitFaster.Caching/Lru/TLruLongTicksPolicy.cs
index 48868977..fb64ca3e 100644
--- a/BitFaster.Caching/Lru/TLruLongTicksPolicy.cs
+++ b/BitFaster.Caching/Lru/TLruLongTicksPolicy.cs
@@ -8,6 +8,7 @@ namespace BitFaster.Caching.Lru
     /// recently used items first, and any item that has expired.
     /// </summary>
     public readonly struct TLruLongTicksPolicy<K, V> : IItemPolicy<K, V, LongTickCountLruItem<K, V>>
+        where K : notnull
     {
         private readonly Duration timeToLive;
 
diff --git a/BitFaster.Caching/Lru/TelemetryPolicy.cs b/BitFaster.Caching/Lru/TelemetryPolicy.cs
index 3cb94d2f..37b95742 100644
--- a/BitFaster.Caching/Lru/TelemetryPolicy.cs
+++ b/BitFaster.Caching/Lru/TelemetryPolicy.cs
@@ -11,6 +11,7 @@ namespace BitFaster.Caching.Lru
     /// <typeparam name="V">The type of the value</typeparam>
     [DebuggerDisplay("Hit = {Hits}, Miss = {Misses}, Upd = {Updated}, Evict = {Evicted}")]
     public struct TelemetryPolicy<K, V> : ITelemetryPolicy<K, V>
+        where K : notnull
     {
         private Counter hitCount;
         private Counter missCount;
diff --git a/BitFaster.Caching/Lru/TickCountLruItem.cs b/BitFaster.Caching/Lru/TickCountLruItem.cs
index 3fae631a..956e4b7c 100644
--- a/BitFaster.Caching/Lru/TickCountLruItem.cs
+++ b/BitFaster.Caching/Lru/TickCountLruItem.cs
@@ -8,6 +8,7 @@ namespace BitFaster.Caching.Lru
     /// <typeparam name="K">The type of the key.</typeparam>
     /// <typeparam name="V">The type of the value.</typeparam>
     public class TickCountLruItem<K, V> : LruItem<K, V>
+        where K : notnull
     {
         /// <summary>
         /// Initializes a new instance of the TickCountLruItem class with the specified key and value.
diff --git a/BitFaster.Caching/Lru/TimeStampedLruItem.cs b/BitFaster.Caching/Lru/TimeStampedLruItem.cs
index b05fc3c1..ac60ebe4 100644
--- a/BitFaster.Caching/Lru/TimeStampedLruItem.cs
+++ b/BitFaster.Caching/Lru/TimeStampedLruItem.cs
@@ -8,6 +8,7 @@ namespace BitFaster.Caching.Lru
     /// <typeparam name="K">The type of the key.</typeparam>
     /// <typeparam name="V">The type of the value.</typeparam>
     public class TimeStampedLruItem<K, V> : LruItem<K, V>
+        where K : notnull
     {
         /// <summary>
         /// Initializes a new instance of the TimeStampedLruItem class with the specified key and value.
diff --git a/BitFaster.Caching/Lru/TlruDateTimePolicy.cs b/BitFaster.Caching/Lru/TlruDateTimePolicy.cs
index effdab1f..81e3a078 100644
--- a/BitFaster.Caching/Lru/TlruDateTimePolicy.cs
+++ b/BitFaster.Caching/Lru/TlruDateTimePolicy.cs
@@ -8,6 +8,7 @@ namespace BitFaster.Caching.Lru
     /// recently used items first, and any item that has expired.
     /// </summary>
     public readonly struct TLruDateTimePolicy<K, V> : IItemPolicy<K, V, TimeStampedLruItem<K, V>>
+        where K : notnull
     {
         private readonly TimeSpan timeToLive;
 
diff --git a/BitFaster.Caching/Lru/TlruTicksPolicy.cs b/BitFaster.Caching/Lru/TlruTicksPolicy.cs
index e800cfa1..1ce6bd5a 100644
--- a/BitFaster.Caching/Lru/TlruTicksPolicy.cs
+++ b/BitFaster.Caching/Lru/TlruTicksPolicy.cs
@@ -13,6 +13,7 @@ namespace BitFaster.Caching.Lru
     /// value will wrap and time measurement will become invalid.
     /// </remarks>
     public readonly struct TLruTicksPolicy<K, V> : IItemPolicy<K, V, TickCountLruItem<K, V>>
+        where K : notnull
     {
         private readonly int timeToLive;