diff --git a/exercises/practice/isbn-verifier/.meta/tests.toml b/exercises/practice/isbn-verifier/.meta/tests.toml
index 6d5a84599..17e18d47a 100644
--- a/exercises/practice/isbn-verifier/.meta/tests.toml
+++ b/exercises/practice/isbn-verifier/.meta/tests.toml
@@ -30,6 +30,12 @@ description = "invalid character in isbn is not treated as zero"
 [28025280-2c39-4092-9719-f3234b89c627]
 description = "X is only valid as a check digit"
 
+[8005b57f-f194-44ee-88d2-a77ac4142591]
+description = "only one check digit is allowed"
+
+[fdb14c99-4cf8-43c5-b06d-eb1638eff343]
+description = "X is not substituted by the value 10"
+
 [f6294e61-7e79-46b3-977b-f48789a4945b]
 description = "valid isbn without separating dashes"
 
diff --git a/exercises/practice/isbn-verifier/IsbnVerifierTests.cs b/exercises/practice/isbn-verifier/IsbnVerifierTests.cs
index 49f20d7b1..a13b6f568 100644
--- a/exercises/practice/isbn-verifier/IsbnVerifierTests.cs
+++ b/exercises/practice/isbn-verifier/IsbnVerifierTests.cs
@@ -42,6 +42,18 @@ public void X_is_only_valid_as_a_check_digit()
         Assert.False(IsbnVerifier.IsValid("3-598-2X507-9"));
     }
 
+    [Fact(Skip = "Remove this Skip property to run this test")]
+    public void Only_one_check_digit_is_allowed()
+    {
+        Assert.False(IsbnVerifier.IsValid("3-598-21508-96"));
+    }
+
+    [Fact(Skip = "Remove this Skip property to run this test")]
+    public void X_is_not_substituted_by_the_value_10()
+    {
+        Assert.False(IsbnVerifier.IsValid("3-598-2X507-5"));
+    }
+
     [Fact(Skip = "Remove this Skip property to run this test")]
     public void Valid_isbn_without_separating_dashes()
     {