diff --git a/ports/webassembly/README.md b/ports/webassembly/README.md
index 9ad04ccb98..89b8b1def7 100644
--- a/ports/webassembly/README.md
+++ b/ports/webassembly/README.md
@@ -70,7 +70,7 @@ something to stdout.  The following code demonstrates basic functionality:
     <script>
       document.addEventListener("micropython-print", function(e) {
         let output = document.getElementById("micropython-stdout");
-        output.innerText += e.data;
+        output.innerText += e.detail;
       }, false);
 
       var mp_js_startup = Module["onRuntimeInitialized"];
diff --git a/ports/webassembly/library.js b/ports/webassembly/library.js
index 6d9b07cfff..fb31b0fe56 100644
--- a/ports/webassembly/library.js
+++ b/ports/webassembly/library.js
@@ -33,8 +33,7 @@ mergeInto(LibraryManager.library, {
                 process.stdout.write(b);
             } else {
                 var c = String.fromCharCode(getValue(ptr + i, 'i8'));
-                var printEvent = new CustomEvent('micropython-print');
-                printEvent.data = c;
+                var printEvent = new CustomEvent('micropython-print', { detail: c });
                 document.dispatchEvent(printEvent);
             }
         }