diff --git a/CredentialCache.Test/NullPropertyTests.cs b/CredentialCache.Test/NullPropertyTests.cs
index ffbb2f7..7575449 100644
--- a/CredentialCache.Test/NullPropertyTests.cs
+++ b/CredentialCache.Test/NullPropertyTests.cs
@@ -65,10 +65,13 @@ public void SerializeRejectsANullNonNullableProperty()
 	}
 
 	[TestMethod]
-	public void AddOrReplaceRejectsANullNonNullablePropertyAndLeavesTheStoreAndCacheUnchanged()
+	[DataRow(false)]
+	[DataRow(true)]
+	public void AddOrReplaceRejectsANullNonNullablePropertyAndLeavesTheStoreAndCacheUnchanged(bool inMemory)
 	{
-		RawBlobCredentialStore store = new();
-		using CredentialCache cache = new(store);
+		RawBlobCredentialStore rawStore = new();
+		InMemoryCredentialStore memoryStore = new();
+		using CredentialCache cache = inMemory ? new(memoryStore) : new(rawStore);
 		PersonaGUID persona = CredentialCache.CreatePersonaGUID();
 		CredentialWithUsernamePassword credential = new()
 		{
@@ -78,7 +81,31 @@ public void AddOrReplaceRejectsANullNonNullablePropertyAndLeavesTheStoreAndCache
 
 		Assert.ThrowsExactly<ArgumentException>(() => cache.AddOrReplace(persona, credential));
 
-		Assert.IsFalse(store.Blobs.ContainsKey(persona));
+		Assert.IsFalse(rawStore.Blobs.ContainsKey(persona));
+		Assert.IsFalse(memoryStore.TryLoad(persona, out _));
 		Assert.IsFalse(cache.TryGet(persona, out _));
 	}
+
+	[TestMethod]
+	public void InMemoryStoreSaveRejectsANullNonNullablePropertyAsAPlatformStoreWould()
+	{
+		InMemoryCredentialStore store = new();
+		PersonaGUID persona = CredentialCache.CreatePersonaGUID();
+
+		Assert.ThrowsExactly<ArgumentException>(() => store.Save(persona, new CredentialWithToken { Token = null! }));
+
+		Assert.IsFalse(store.TryLoad(persona, out _));
+	}
+
+	[TestMethod]
+	public void InMemoryStoreStillSavesACompleteCredential()
+	{
+		using CredentialCache cache = new(new InMemoryCredentialStore());
+		PersonaGUID persona = CredentialCache.CreatePersonaGUID();
+
+		cache.AddOrReplace(persona, new CredentialWithToken { Token = SemanticString<CredentialToken>.Create("abc") });
+
+		Assert.IsTrue(cache.TryGet(persona, out Credential? credential));
+		Assert.AreEqual("abc", ((CredentialWithToken)credential!).Token.ToString());
+	}
 }
diff --git a/CredentialCache/CredentialCache.cs b/CredentialCache/CredentialCache.cs
index 0d3a179..891d982 100644
--- a/CredentialCache/CredentialCache.cs
+++ b/CredentialCache/CredentialCache.cs
@@ -212,8 +212,9 @@ public bool TryGet(PersonaGUID persona, out Credential? credential)
 	/// cache and the store are left unchanged.
 	/// </exception>
 	/// <exception cref="ArgumentException">
-	/// The store serializes credentials and <paramref name="credential"/> has a non-nullable
-	/// property set to <c>null</c>. The cache and the store are left unchanged.
+	/// <paramref name="credential"/> has a non-nullable property set to <c>null</c>, so no
+	/// serializing store could read it back. The cache and the store are left unchanged, whichever
+	/// store is configured.
 	/// </exception>
 	public void AddOrReplace(PersonaGUID persona, Credential credential)
 	{
@@ -221,9 +222,10 @@ public void AddOrReplace(PersonaGUID persona, Credential credential)
 		ArgumentNullException.ThrowIfNull(credential);
 		ThrowIfDisposed();
 
-		// Checked here rather than left to the store, so every store refuses the same types. The
-		// in-memory store never serializes, and it accepted types every platform store rejects.
-		CredentialSerialization.ThrowIfUnknownType(credential);
+		// Checked here rather than left to the store, so every store refuses the same credentials.
+		// The in-memory store never serializes, and it accepted unregistered types and null
+		// non-nullable properties that every platform store rejects.
+		CredentialSerialization.ThrowIfNotSerializable(credential);
 
 		lock (LockFor(persona))
 		{
diff --git a/CredentialCache/Storage/CredentialSerialization.cs b/CredentialCache/Storage/CredentialSerialization.cs
index a72b3bc..12a1280 100644
--- a/CredentialCache/Storage/CredentialSerialization.cs
+++ b/CredentialCache/Storage/CredentialSerialization.cs
@@ -166,6 +166,21 @@ internal static void ThrowIfUnknownType(Credential credential)
 		}
 	}
 
+	/// <summary>
+	/// Throws what <see cref="Serialize(Credential)"/> would throw for <paramref name="credential"/>,
+	/// without keeping its serialized form, so a store that never serializes refuses the same
+	/// credentials as one that does.
+	/// </summary>
+	/// <exception cref="CredentialStoreException">
+	/// The credential's type is neither one of the library's own nor registered with
+	/// <see cref="RegisterDerivedType{T}(string)"/>.
+	/// </exception>
+	/// <exception cref="ArgumentException">
+	/// <paramref name="credential"/> has a non-nullable property set to <c>null</c>, so it could
+	/// not be read back.
+	/// </exception>
+	internal static void ThrowIfNotSerializable(Credential credential) => Zero(Serialize(credential));
+
 	/// <summary>
 	/// Serializes the credential to a UTF-8 JSON byte array.
 	/// </summary>
diff --git a/CredentialCache/Storage/InMemoryCredentialStore.cs b/CredentialCache/Storage/InMemoryCredentialStore.cs
index 9d60696..6e36f2b 100644
--- a/CredentialCache/Storage/InMemoryCredentialStore.cs
+++ b/CredentialCache/Storage/InMemoryCredentialStore.cs
@@ -30,7 +30,7 @@ public void Save(PersonaGUID persona, Credential credential)
 
 		// Refuse what a platform store could not serialize, so code tested against this store
 		// sees the same failure it would in production.
-		CredentialSerialization.ThrowIfUnknownType(credential);
+		CredentialSerialization.ThrowIfNotSerializable(credential);
 		_items[persona] = credential;
 	}