diff --git a/reconcile-js/src/core.ts b/reconcile-js/src/core.ts
index 1139dd5..333eda6 100644
--- a/reconcile-js/src/core.ts
+++ b/reconcile-js/src/core.ts
@@ -254,6 +254,7 @@ export interface ReconcileApi {
    * @param diff - The compact diff array (inserts as strings, deletes as negative integers, retained spans as positive integers)
    * @param tokenizer - The tokenisation strategy, which is the same as used in `reconcile`.
    * @returns The reconstructed changed text as a string.
+   * @throws {Error} If the diff contains a non-integer, or spans more characters than `original` has.
    */
   undiff(
     original: string,
diff --git a/reconcile-js/src/index.test.ts b/reconcile-js/src/index.test.ts
index e88f2ed..4d9efb5 100644
--- a/reconcile-js/src/index.test.ts
+++ b/reconcile-js/src/index.test.ts
@@ -91,6 +91,27 @@ describe.each(backends)('reconcile [$name]', ({ api }) => {
     expect(withBigints.some((item) => typeof item === 'bigint')).toBe(true);
     expect(undiff(original, withBigints)).toEqual(changed);
   });
+
+  // A trap would also be an `Error` (`RuntimeError: unreachable`), so check the message
+  it.each([
+    { invalidDiff: [10], message: 'Invalid diff' },
+    { invalidDiff: [2 ** 32], message: 'Invalid diff' },
+    { invalidDiff: [-(2 ** 40)], message: 'Invalid diff' },
+    { invalidDiff: [2n ** 40n], message: 'Invalid diff' },
+    { invalidDiff: [1.5], message: 'must be an integer' },
+    { invalidDiff: [-0.4, 'X', 3], message: 'must be an integer' },
+    { invalidDiff: [NaN], message: 'NaN' },
+  ])('undiff rejects $invalidDiff with an Error', ({ invalidDiff, message }) => {
+    let thrown: unknown;
+    try {
+      undiff('abc', invalidDiff);
+    } catch (error) {
+      thrown = error;
+    }
+
+    expect(thrown).toBeInstanceOf(Error);
+    expect((thrown as Error).message).toContain(message);
+  });
 });
 
 describe.each(backends)('WASM cleanup after exceptions [$name]', ({ api, bindings }) => {
diff --git a/src/wasm.rs b/src/wasm.rs
index 06cccbe..e7195a1 100644
--- a/src/wasm.rs
+++ b/src/wasm.rs
@@ -81,45 +81,44 @@ pub fn generic_reconcile(
 ///
 /// # Errors
 ///
-/// Returns a JS error if integer overflow occurs during diff computation.
+/// Throws a JS `Error` if integer overflow occurs during diff computation.
 #[wasm_bindgen(js_name = diff)]
 pub fn diff(
     parent: &str,
     changed: &TextWithCursors,
     tokenizer: BuiltinTokenizer,
-) -> Result<Vec<JsValue>, JsValue> {
+) -> Result<Vec<JsValue>, JsError> {
     set_panic_hook();
 
     let edited_text = EditedText::from_strings_with_tokenizer(parent, changed, &*tokenizer);
     edited_text
         .to_diff()
         .map(|diff| diff.into_iter().map(std::convert::Into::into).collect())
-        .map_err(|e| JsValue::from_str(&e.to_string()))
+        .map_err(JsError::from)
 }
 
 /// Inverse of `diff`, applies a compact diff representation to a parent text
 ///
 /// # Errors
 ///
-/// Returns a JS error if the diff format is invalid or references ranges
+/// Throws a JS `Error` if the diff format is invalid or references ranges
 /// exceeding the original text length.
 #[wasm_bindgen(js_name = undiff)]
 pub fn undiff(
     parent: &str,
     diff: Vec<JsValue>,
     tokenizer: BuiltinTokenizer,
-) -> Result<String, JsValue> {
+) -> Result<String, JsError> {
     set_panic_hook();
 
     let parsed_diff: Vec<_> = diff
         .into_iter()
         .map(std::convert::TryInto::try_into)
-        .collect::<Result<_, _>>()
-        .map_err(|e: crate::types::number_or_text::DeserialisationError| -> JsValue { e.into() })?;
+        .collect::<Result<_, crate::types::number_or_text::DeserialisationError>>()?;
 
     EditedText::from_diff(parent, parsed_diff, &*tokenizer)
         .map(|edited_text| edited_text.apply().text())
-        .map_err(|e| JsValue::from_str(&e.to_string()))
+        .map_err(JsError::from)
 }
 
 fn set_panic_hook() {