diff --git a/src/CodingAlphabet.cs b/src/CodingAlphabet.cs
index 37f0e91..f51c77e 100644
--- a/src/CodingAlphabet.cs
+++ b/src/CodingAlphabet.cs
@@ -54,7 +54,7 @@ public CodingAlphabet(int length, string alphabet, bool caseInsensitive = false)
             char c = alphabet[i];
             if (caseInsensitive && char.IsLetter(c))
             {
-                char c2 = char.IsUpper(c) ? char.ToLower(c) : char.ToUpper(c);
+                char c2 = char.IsUpper(c) ? char.ToLowerInvariant(c) : char.ToUpperInvariant(c);
                 if (alphabet.Contains(c2))
                 {
                     throw new ArgumentException($"Case-sensitivity can't be selected with an alphabet that contains both cases of the same letter", nameof(caseInsensitive));
diff --git a/test/CodingAlphabetTest.cs b/test/CodingAlphabetTest.cs
index bed7b19..ec9c361 100644
--- a/test/CodingAlphabetTest.cs
+++ b/test/CodingAlphabetTest.cs
@@ -26,6 +26,10 @@ class DummyAlphabet(string alphabet) : CodingAlphabet(10, alphabet, caseInsensit
     {
     }
 
+    class DummyAutoAlphabet(string alphabet, bool caseInsensitive) : CodingAlphabet(alphabet.Length, alphabet, caseInsensitive)
+    {
+    }
+
     [Test]
     public void Ctor_WithBothCasesOfLettersAndCaseInsensitive_ShouldThrow()
     {
@@ -43,4 +47,18 @@ public void Ctor_ProperArguments_ShouldNotThrow()
     {
         Assert.DoesNotThrow(() => new DummyAlphabet("01234567ab"));
     }
+
+    [Test]
+    [SetCulture("tr-TR")]
+    public void Ctor_ShouldNotThrow_On_Turkish_Locale_With_Lowercase_I()
+    {
+        Assert.DoesNotThrow(() => new DummyAutoAlphabet("i", true));
+    }
+
+    [Test]
+    [SetCulture("tr-TR")]
+    public void Ctor_ShouldNotThrow_On_Turkish_Locale_With_Uppercase_I()
+    {
+        Assert.DoesNotThrow(() => new DummyAutoAlphabet("I", true));
+    }
 }