4 files changed, 13 insertions, 7 deletions
diff --git a/ports/webassembly/proxy_c.c b/ports/webassembly/proxy_c.c
index 3097d6d3a..f8d6d8530 100644
--- a/ports/webassembly/proxy_c.c
+++ b/ports/webassembly/proxy_c.c
@@ -49,6 +49,7 @@ enum {
 };
 
 enum {
+    PROXY_KIND_JS_UNDEFINED = 0,
     PROXY_KIND_JS_NULL = 1,
     PROXY_KIND_JS_BOOLEAN = 2,
     PROXY_KIND_JS_INTEGER = 3,
@@ -78,7 +79,9 @@ static inline mp_obj_t proxy_c_get_obj(uint32_t c_ref) {
 }
 
 mp_obj_t proxy_convert_js_to_mp_obj_cside(uint32_t *value) {
-    if (value[0] == PROXY_KIND_JS_NULL) {
+    if (value[0] == PROXY_KIND_JS_UNDEFINED) {
+        return mp_const_none;
+    } else if (value[0] == PROXY_KIND_JS_NULL) {
         return mp_const_none;
     } else if (value[0] == PROXY_KIND_JS_BOOLEAN) {
         return mp_obj_new_bool(value[1]);
diff --git a/ports/webassembly/proxy_js.js b/ports/webassembly/proxy_js.js
index 042deb7b0..05d0f2da4 100644
--- a/ports/webassembly/proxy_js.js
+++ b/ports/webassembly/proxy_js.js
@@ -38,6 +38,7 @@ const PROXY_KIND_MP_GENERATOR = 7;
 const PROXY_KIND_MP_OBJECT = 8;
 const PROXY_KIND_MP_JSPROXY = 9;
 
+const PROXY_KIND_JS_UNDEFINED = 0;
 const PROXY_KIND_JS_NULL = 1;
 const PROXY_KIND_JS_BOOLEAN = 2;
 const PROXY_KIND_JS_INTEGER = 3;
@@ -109,7 +110,9 @@ function proxy_call_python(target, argumentsList) {
 
 function proxy_convert_js_to_mp_obj_jsside(js_obj, out) {
     let kind;
-    if (js_obj === null) {
+    if (js_obj === undefined) {
+        kind = PROXY_KIND_JS_UNDEFINED;
+    } else if (js_obj === null) {
         kind = PROXY_KIND_JS_NULL;
     } else if (typeof js_obj === "boolean") {
         kind = PROXY_KIND_JS_BOOLEAN;
@@ -185,7 +188,7 @@ function proxy_convert_mp_to_js_obj_jsside(value) {
     }
     if (kind === PROXY_KIND_MP_NONE) {
         // None
-        obj = null;
+        obj = undefined;
     } else if (kind === PROXY_KIND_MP_BOOL) {
         // bool
         obj = Module.getValue(value + 4, "i32") ? true : false;
diff --git a/tests/ports/webassembly/py_proxy_to_js.mjs.exp b/tests/ports/webassembly/py_proxy_to_js.mjs.exp
index 279df7bdf..3fd38c2b5 100644
--- a/tests/ports/webassembly/py_proxy_to_js.mjs.exp
+++ b/tests/ports/webassembly/py_proxy_to_js.mjs.exp
@@ -1,4 +1,4 @@
 false 1
 true [ 1, 2, 3 ]
-true [ null, true, 1.2 ]
-true { tuple: [ 1, 2, 3 ], one: 1, list: [ null, true, 1.2 ] }
+true [ undefined, true, 1.2 ]
+true { tuple: [ 1, 2, 3 ], one: 1, list: [ undefined, true, 1.2 ] }
diff --git a/tests/ports/webassembly/run_python_async.mjs.exp b/tests/ports/webassembly/run_python_async.mjs.exp
index f441bc5cf..827f853b9 100644
--- a/tests/ports/webassembly/run_python_async.mjs.exp
+++ b/tests/ports/webassembly/run_python_async.mjs.exp
@@ -23,7 +23,7 @@ py 1
 setTimeout resolved
 resolved value: 123
 py 2
-2 null
+2 undefined
 = TEST 4 ==========
 1
 py 1
@@ -35,4 +35,4 @@ py 3
 setTimeout B resolved
 resolved value: 456
 py 4
-2 null
+2 undefined