diff --git a/BUILD b/BUILD
index 2ad94c583d..46f4b08dcf 100644
--- a/BUILD
+++ b/BUILD
@@ -439,6 +439,25 @@ upb_proto_library(
     deps = [":msg_test_proto"],
 )
 
+proto_library(
+    name = "proto3_test_proto",
+    testonly = 1,
+    srcs = ["upb/proto3_test.proto"],
+    deps = ["@com_google_protobuf//:descriptor_proto"],
+)
+
+upb_proto_library(
+    name = "proto3_test_upb_proto",
+    testonly = 1,
+    deps = [":proto3_test_proto"],
+)
+
+upb_proto_reflection_library(
+    name = "proto3_test_upb_proto_reflection",
+    testonly = 1,
+    deps = [":proto3_test_proto"],
+)
+
 upb_proto_reflection_library(
     name = "msg_test_upb_proto_reflection",
     testonly = 1,
@@ -471,6 +490,18 @@ upb_proto_reflection_library(
     deps = ["@com_google_protobuf//:timestamp_proto"],
 )
 
+cc_test(
+    name = "proto3_test",
+    srcs = ["upb/proto3_test.cc"],
+    copts = UPB_DEFAULT_CPPOPTS,
+    deps = [
+        ":proto3_test_upb_proto",
+        ":proto3_test_upb_proto_reflection",
+        ":reflection",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
 cc_test(
     name = "test_cpp",
     srcs = ["upb/test_cpp.cc"],
diff --git a/upb/def.c b/upb/def.c
index 2f216bbfb9..4c855d001e 100644
--- a/upb/def.c
+++ b/upb/def.c
@@ -2515,7 +2515,7 @@ static void create_fielddef(
     CHK_OOM(
         upb_strtable_insert(&oneof->ntof, name.data, name.size, v, ctx->arena));
   } else {
-    if (f->proto3_optional_) {
+    if (f->proto3_optional_ && !is_extension) {
       symtab_errf(ctx, "field with proto3_optional was not in a oneof (%s)",
                   f->full_name);
     }
diff --git a/upb/proto3_test.cc b/upb/proto3_test.cc
new file mode 100644
index 0000000000..2d12d7bb4f
--- /dev/null
+++ b/upb/proto3_test.cc
@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2009-2021, Google LLC
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of Google LLC nor the
+ *       names of its contributors may be used to endorse or promote products
+ *       derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL Google LLC BE LIABLE FOR ANY DIRECT,
+ * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "gtest/gtest.h"
+#include "upb/proto3_test.upb.h"
+#include "upb/proto3_test.upbdefs.h"
+#include "upb/def.hpp"
+
+TEST(Proto3Test, SyntheticOneofExtension) {
+  upb::SymbolTable defpool;
+  upb::MessageDefPtr md(upb_test_TestMessage3_getmsgdef(defpool.ptr()));
+  ASSERT_EQ(md.field_count(), 6);
+}
diff --git a/upb/proto3_test.proto b/upb/proto3_test.proto
new file mode 100644
index 0000000000..f83a8b8c4b
--- /dev/null
+++ b/upb/proto3_test.proto
@@ -0,0 +1,22 @@
+syntax = "proto3";
+
+package upb.test;
+
+import "google/protobuf/descriptor.proto";
+
+extend google.protobuf.MessageOptions {
+  optional string my_option = 51235;
+}
+
+message MyMessage3 {
+  option (my_option) = "Hello world!";
+}
+
+message TestMessage3 {
+  optional int32 i32 = 1;
+  repeated int32 r_i32 = 2;
+  optional string str = 3;
+  repeated string r_str = 4;
+  optional TestMessage3 msg = 5;
+  repeated TestMessage3 r_msg = 6;
+}