diff --git a/src/Zora.res b/src/Zora.res
index 13ab34e..66634b2 100644
--- a/src/Zora.res
+++ b/src/Zora.res
@@ -20,7 +20,7 @@ type testMessage = string
 @send external equal: (t, 't, 't, ~msg: testMessage=?) => unit = "equal"
 @send external notEqual: (t, 't, 't, ~msg: testMessage=?) => unit = "notEqual"
 @send external is: (t, 't, 't, ~msg: testMessage=?) => unit = "is"
-@send external isNot: (t, 't, 't, testMessage) => unit = "isNot"
+@send external isNot: (t, 't, 't, ~msg: testMessage=?) => unit = "isNot"
 @send external ok: (t, bool, testMessage) => unit = "ok"
 @send external notOk: (t, bool, testMessage) => unit = "notOk"
 @send external fail: (t, testMessage) => unit = "fail"
diff --git a/tests/assertions.test.mjs b/tests/assertions.test.mjs
index b33944b..08eb609 100644
--- a/tests/assertions.test.mjs
+++ b/tests/assertions.test.mjs
@@ -41,7 +41,11 @@ Zora$1.test("Test assertions", (async function (t) {
                 var x = {
                   hello: "world"
                 };
+                var z = {
+                  hello: "world"
+                };
                 t.is(x, x);
+                t.isNot(x, z);
               }));
       }));
 
diff --git a/tests/assertions.test.res b/tests/assertions.test.res
index 032b6c6..61385a6 100644
--- a/tests/assertions.test.res
+++ b/tests/assertions.test.res
@@ -9,7 +9,7 @@ zora("Test assertions", async t => {
     let z = {"hello": "world"}
     t->is(x, x, ~msg="object is object")
     t->is(x, y, ~msg="object is object")
-    t->isNot(x, z, "object is not object with same values")
+    t->isNot(x, z, ~msg="object is not object with same values")
     t->equal(x, z, ~msg="Object is deep equal")
     t->ok(true, "boolean is ok")
     t->notOk(false, "boolean is not ok")
@@ -26,10 +26,9 @@ zora("Test assertions", async t => {
     t->equal(42, 42)
     t->notEqual(42, 43)
     let x = {"hello": "world"}
-    // let z = {"hello": "world"}
+    let z = {"hello": "world"}
     t->is(x, x)
-    // t->isNot(x, z)
-    // t->equal(x, z)
+    t->isNot(x, z)
     // t->ok(true)
     // t->notOk(false)
     // t->optionNone(None)