diff --git a/packages/exo/src/exo-tools.js b/packages/exo/src/exo-tools.js
index 41b83a04be..56fd439968 100644
--- a/packages/exo/src/exo-tools.js
+++ b/packages/exo/src/exo-tools.js
@@ -57,8 +57,8 @@ const PassableMethodGuard = M.call().rest(M.any()).returns(M.any());
  * @param {Passable[]} syncArgs
  * @param {MatchConfig} matchConfig
  * @param {string} [label]
- * @returns {Passable[]} Returns the args that should be passed to the
- * raw method
+ * @returns {import('@endo/pass-style').Passable[]}
+ * Returns the args that should be passed to the raw method.
  */
 const defendSyncArgs = (syncArgs, matchConfig, label = undefined) => {
   const {
@@ -69,7 +69,8 @@ const defendSyncArgs = (syncArgs, matchConfig, label = undefined) => {
     redactedIndices,
   } = matchConfig;
 
-  // Use syncArgs if possible, but copy it when necessary to implement redactions.
+  // Use syncArgs if possible, but copy it when necessary to implement
+  // redactions.
   let matchableArgs = syncArgs;
   if (restArgGuardIsRaw && syncArgs.length > declaredLen) {
     const restLen = syncArgs.length - declaredLen;
@@ -96,11 +97,11 @@ const defendSyncArgs = (syncArgs, matchConfig, label = undefined) => {
   if (hasRestArgGuard) {
     return syncArgs;
   }
-  if (syncArgs.length <= declaredLen) {
-    return syncArgs;
-  }
-  // Ignore extraneous arguments, as a JS function call would do.
-  return syncArgs.slice(0, declaredLen);
+  syncArgs.length <= declaredLen ||
+    Fail`${q(label)} accepts at most ${q(declaredLen)} arguments, not ${q(
+      syncArgs.length,
+    )}: ${syncArgs}`;
+  return syncArgs;
 };
 
 /**
diff --git a/packages/exo/test/test-heap-classes.js b/packages/exo/test/test-heap-classes.js
index c6c3525cf7..5d5206f4f8 100644
--- a/packages/exo/test/test-heap-classes.js
+++ b/packages/exo/test/test-heap-classes.js
@@ -16,12 +16,15 @@ const NoExtraI = M.interface('NoExtra', {
 });
 
 test('what happens with extra arguments', t => {
-  const exo = makeExo('WithExtra', NoExtraI, {
+  const exo = makeExo('NoExtraArgs', NoExtraI, {
     foo(x) {
       t.is(x, undefined);
     },
   });
-  exo.foo('an extra arg');
+  t.throws(() => exo.foo('an extra arg'), {
+    message:
+      '"In \\"foo\\" method of (NoExtraArgs)" accepts at most 0 arguments, not 1: ["an extra arg"]',
+  });
 });
 
 const OptionalArrayI = M.interface('OptionalArray', {