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.fs b/exercises/practice/isbn-verifier/IsbnVerifierTests.fs
index 342de3e08..5b3a03714 100644
--- a/exercises/practice/isbn-verifier/IsbnVerifierTests.fs
+++ b/exercises/practice/isbn-verifier/IsbnVerifierTests.fs
@@ -33,6 +33,14 @@ let ``Invalid character in isbn is not treated as zero`` () =
 let ``X is only valid as a check digit`` () =
     isValid "3-598-2X507-9" |> should equal false
 
+[<Fact(Skip = "Remove this Skip property to run this test")>]
+let ``Only one check digit is allowed`` () =
+    isValid "3-598-21508-96" |> should equal false
+
+[<Fact(Skip = "Remove this Skip property to run this test")>]
+let ``X is not substituted by the value 10`` () =
+    isValid "3-598-2X507-5" |> should equal false
+
 [<Fact(Skip = "Remove this Skip property to run this test")>]
 let ``Valid isbn without separating dashes`` () =
     isValid "3598215088" |> should equal true