diff --git a/crates/cli-support/src/js/mod.rs b/crates/cli-support/src/js/mod.rs
index 618b4282dc9..35bba115fdc 100644
--- a/crates/cli-support/src/js/mod.rs
+++ b/crates/cli-support/src/js/mod.rs
@@ -2123,7 +2123,7 @@ impl<'a> Context<'a> {
         self.global(
             "
             function _assertChar(c) {
-                if (typeof(c) !== 'number' || c >= 0x110000 || (c >= 0xD800 && c < 0xE000)) throw new Error(`expected a number argument that is a valid Unicode scalar value, found ${c}`);
+                if (typeof(c) === 'number' && (c >= 0x110000 || (c >= 0xD800 && c < 0xE000))) throw new Error(`expected a valid Unicode scalar value, found ${c}`);
             }
             ",
         );
diff --git a/tests/wasm/char.js b/tests/wasm/char.js
index c9e676023e0..a1e7d83e4cd 100644
--- a/tests/wasm/char.js
+++ b/tests/wasm/char.js
@@ -7,6 +7,7 @@ exports.js_option_identity = a => a;
 exports.js_works = () => {
     assert.strictEqual(wasm.letter(), 'a');
     assert.strictEqual(wasm.face(), '😀');
+    assert.strictEqual(wasm.rust_identity(''), '\u0000');
     assert.strictEqual(wasm.rust_identity('Ղ'), 'Ղ');
     assert.strictEqual(wasm.rust_identity('ҝ'), 'ҝ');
     assert.strictEqual(wasm.rust_identity('Δ'), 'Δ');
@@ -16,6 +17,12 @@ exports.js_works = () => {
     wasm.rust_letter('a');
     wasm.rust_face('😀');
 
-    assert.throws(() => wasm.rust_js_identity('\uD83D'), /expected a number argument that is a valid Unicode scalar value, found 55357/);
-    assert.throws(() => wasm.rust_js_option_identity('\uD83D'), /expected a number argument that is a valid Unicode scalar value, found 55357/);
+    assert.strictEqual(wasm.rust_option_identity(undefined), undefined);
+    assert.strictEqual(wasm.rust_option_identity(null), undefined);
+    assert.strictEqual(wasm.rust_option_identity(''), '\u0000');
+    assert.strictEqual(wasm.rust_option_identity('\u0000'), '\u0000');
+
+    assert.throws(() => wasm.rust_identity(55357), /c.codePointAt is not a function/);
+    assert.throws(() => wasm.rust_identity('\uD83D'), /expected a valid Unicode scalar value, found 55357/);
+    assert.throws(() => wasm.rust_option_identity('\uD83D'), /expected a valid Unicode scalar value, found 55357/);
 };
diff --git a/tests/wasm/char.rs b/tests/wasm/char.rs
index b23dd94306d..d942b0ff5af 100644
--- a/tests/wasm/char.rs
+++ b/tests/wasm/char.rs
@@ -13,6 +13,11 @@ pub fn rust_identity(c: char) -> char {
     c
 }
 
+#[wasm_bindgen]
+pub fn rust_option_identity(c: Option<char>) -> Option<char> {
+    c
+}
+
 #[wasm_bindgen]
 pub fn rust_js_identity(c: char) -> char {
     js_identity(c)