diff --git a/LicenseHeaderManager.Core.Tests/LicenseHeaderReplacerTest.cs b/LicenseHeaderManager.Core.Tests/LicenseHeaderReplacerTest.cs
index 845726a..cbcc28a 100644
--- a/LicenseHeaderManager.Core.Tests/LicenseHeaderReplacerTest.cs
+++ b/LicenseHeaderManager.Core.Tests/LicenseHeaderReplacerTest.cs
@@ -64,6 +64,16 @@ public void GetLanguageFromExtension_LanguagesAreEmpty_ReturnsNull ()
       Assert.That (language, Is.Null);
     }
 
+    [Test]
+    public void GetLanguageFromExtension_NullExtension_ReturnsNull()
+    {
+      var replacer = new LicenseHeaderReplacer(_languages, Enumerable.Empty<string>());
+
+      var language = replacer.GetLanguageFromExtension(null);
+
+      Assert.That(language, Is.Null);
+    }
+
     [Test]
     public void GetLanguageFromExtension_LanguagesAreNull_DoesNotThrowExceptionAndReturnsNull ()
     {
diff --git a/LicenseHeaderManager.Core/LicenseHeaderReplacer.cs b/LicenseHeaderManager.Core/LicenseHeaderReplacer.cs
index f3170ca..2103c45 100644
--- a/LicenseHeaderManager.Core/LicenseHeaderReplacer.cs
+++ b/LicenseHeaderManager.Core/LicenseHeaderReplacer.cs
@@ -62,6 +62,9 @@ protected LicenseHeaderReplacer ()
 
     public Language GetLanguageFromExtension (string extension)
     {
+      if (extension == null)
+        return null;
+
       return _languages?.FirstOrDefault (x => x.Extensions.Any (y => extension.EndsWith (y, StringComparison.OrdinalIgnoreCase)));
     }