diff --git a/Extensions.Test/EnumerableExtensionsTests.cs b/Extensions.Test/EnumerableExtensionsTests.cs
index 459a5bc..6e186ce 100644
--- a/Extensions.Test/EnumerableExtensionsTests.cs
+++ b/Extensions.Test/EnumerableExtensionsTests.cs
@@ -340,6 +340,21 @@ public void JoinThrowsArgumentNullExceptionWhenSeparatorIsNull()
 		Assert.ThrowsExactly<ArgumentNullException>(() => items.Join(separator));
 	}
 
+	[TestMethod]
+	[DataRow(NullItemHandling.Remove)]
+	[DataRow(NullItemHandling.Include)]
+	[DataRow(NullItemHandling.Throw)]
+	public void JoinWithNullItemHandlingThrowsArgumentNullExceptionWhenSeparatorIsNull(NullItemHandling nullItemHandling)
+	{
+		// Arrange
+		List<string> items = ["a", "b", "c"];
+		string separator = null!;
+
+		// Act & Assert
+		ArgumentNullException exception = Assert.ThrowsExactly<ArgumentNullException>(() => items.Join(separator, nullItemHandling));
+		Assert.AreEqual("separator", exception.ParamName);
+	}
+
 	[TestMethod]
 	public void JoinWithNullItemHandlingRemoveRemovesNullItems()
 	{
diff --git a/Extensions/EnumerableExtensions.cs b/Extensions/EnumerableExtensions.cs
index 636c125..1e5c637 100644
--- a/Extensions/EnumerableExtensions.cs
+++ b/Extensions/EnumerableExtensions.cs
@@ -259,6 +259,13 @@ public static string Join<T>(this IEnumerable<T> items, string separator, NullIt
 		}
 #pragma warning restore KTSU0004 // Use Ensure.NotNull instead of manual null check
 
+#pragma warning disable KTSU0004 // Use Ensure.NotNull instead of manual null check
+		if (separator is null)
+		{
+			throw new ArgumentNullException(nameof(separator), "Separator cannot be null.");
+		}
+#pragma warning restore KTSU0004 // Use Ensure.NotNull instead of manual null check
+
 		if (nullItemHandling is NullItemHandling.Throw)
 		{
 			if (items.AnyNull())