diff --git a/ChangeLog.md b/ChangeLog.md
index aedfee2d08372..806581db20faa 100644
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -22,8 +22,10 @@ Current Trunk
   will need to wait on the returned Promise, using `await` or its `then`
   callback, to get the module instance (#10697). This fixes some long-standing
   bugs with that option which have been reported multiple times, but is a
-  breaking change - sorry about that. See detailed examples for the
-  current usage in `src/settings.js` on `MODULARIZE`.
+  breaking change - sorry about that. To reduce the risk of confusing breakage,
+  in a build with `ASSERTIONS` we will show a clear warning on common errors.
+  For more, see detailed examples for the current usage in `src/settings.js` on
+  `MODULARIZE`.
 - A new `PRINTF_LONG_DOUBLE` option allows printf to print long doubles at full
   float128 precision. (#11130)
 - `emscripten_async_queue_on_thread` has been renamed to
diff --git a/src/parseTools.js b/src/parseTools.js
index b493280b2e5ef..26455d426c56d 100644
--- a/src/parseTools.js
+++ b/src/parseTools.js
@@ -1708,3 +1708,26 @@ function sendI64Argument(low, high) {
     return low + ', ' + high;
   }
 }
+
+// Add assertions to catch common errors when using the Promise object we
+// create on Module.ready() and return from MODULARIZE Module() invocations.
+function addReadyPromiseAssertions(promise) {
+  // Warn on someone doing
+  //
+  //  var instance = Module();
+  //  ...
+  //  instance._main();
+  var properties = keys(EXPORTED_FUNCTIONS);
+  // Also warn on onRuntimeInitialized which might be a common pattern with
+  // older MODULARIZE-using codebases.
+  properties.push('onRuntimeInitialized');
+  return properties.map(function(property) {
+    const warningEnding = `${property} on the Promise object, instead of the instance. Use .then() to get called back with the instance, see the MODULARIZE docs in src/settings.js`;
+    return `
+      if (!Object.getOwnPropertyDescriptor(${promise}, '${property}')) {
+        Object.defineProperty(${promise}, '${property}', { configurable: true, get: function() { abort('You are getting ${warningEnding}') } });
+        Object.defineProperty(${promise}, '${property}', { configurable: true, set: function() { abort('You are setting ${warningEnding}') } });
+      }
+    `;
+  }).join('\n');
+}
diff --git a/src/shell.js b/src/shell.js
index d820d97fe4d3a..8a3f1994d8979 100644
--- a/src/shell.js
+++ b/src/shell.js
@@ -45,6 +45,9 @@ Module['ready'] = new Promise(function(resolve, reject) {
   readyPromiseResolve = resolve;
   readyPromiseReject = reject;
 });
+#if ASSERTIONS
+{{{ addReadyPromiseAssertions("Module['ready']") }}}
+#endif
 #endif
 
 // --pre-jses are emitted after the Module integration code, so that they can
diff --git a/src/shell_minimal.js b/src/shell_minimal.js
index 0ab58587447fd..4f62340357253 100644
--- a/src/shell_minimal.js
+++ b/src/shell_minimal.js
@@ -22,6 +22,9 @@ Module['ready'] = new Promise(function(resolve, reject) {
   readyPromiseResolve = resolve;
   readyPromiseReject = reject;
 });
+#if ASSERTIONS
+{{{ addReadyPromiseAssertions("Module['ready']") }}}
+#endif
 #endif
 
 #if ENVIRONMENT_MAY_BE_NODE
diff --git a/tests/test_other.py b/tests/test_other.py
index c281d46722687..06c2426fe056e 100644
--- a/tests/test_other.py
+++ b/tests/test_other.py
@@ -10386,6 +10386,29 @@ def test_assertions_on_outgoing_module_api_changes(self):
 Module.arguments has been replaced with plain arguments_
 ''', run_js('a.out.js', assert_returncode=None, stderr=PIPE))
 
+  def test_assertions_on_ready_promise(self):
+    # check that when assertions are on we give useful error messages for
+    # mistakenly thinking the Promise is an instance. I.e., once you could do
+    # Module()._main to get an instance and the main function, but after
+    # the breaking change in #10697 Module() now returns a promise, and to get
+    # the instance you must use .then() to get a callback with the instance.
+    create_test_file('test.js', r'''
+      try {
+        Module()._main;
+      } catch(e) {
+        console.log(e);
+      }
+      try {
+        Module().onRuntimeInitialized = 42;
+      } catch(e) {
+        console.log(e);
+      }
+    ''')
+    run_process([PYTHON, EMCC, path_from_root('tests', 'hello_world.c'), '-s', 'MODULARIZE', '-s', 'ASSERTIONS', '--extern-post-js', 'test.js'])
+    out = run_js('a.out.js')
+    self.assertContained('You are getting _main on the Promise object, instead of the instance. Use .then() to get called back with the instance, see the MODULARIZE docs in src/settings.js', out)
+    self.assertContained('You are setting onRuntimeInitialized on the Promise object, instead of the instance. Use .then() to get called back with the instance, see the MODULARIZE docs in src/settings.js', out)
+
   def test_em_asm_duplicate_strings(self):
     # We had a regression where tow different EM_ASM strings from two diffferent
     # object files were de-duplicated in wasm-emscripten-finalize.  This used to