diff --git a/tests/bindings/lua/test.proto b/tests/bindings/lua/test.proto
index 4c3b64edb5..c751436016 100644
--- a/tests/bindings/lua/test.proto
+++ b/tests/bindings/lua/test.proto
@@ -32,3 +32,38 @@ message TestLargeFieldNumber {
 message TestTimestamp {
   optional google.protobuf.Timestamp ts = 1;
 }
+
+message HelloRequest {
+  optional uint32 id = 1;
+  optional uint32 random_name_a0 = 2;
+  optional uint32 random_name_a1 = 3;
+  optional uint32 random_name_a2 = 4;
+  optional uint32 random_name_a3 = 5;
+  optional uint32 random_name_a4 = 6;
+  optional uint32 random_name_a5 = 7;
+  optional uint32 random_name_a6 = 8;
+  optional uint32 random_name_a7 = 9;
+  optional uint32 random_name_a8 = 10;
+  optional uint32 random_name_a9 = 11;
+  optional uint32 random_name_b0 = 12;
+  optional uint32 random_name_b1 = 13;
+  optional uint32 random_name_b2 = 14;
+  optional uint32 random_name_b3 = 15;
+  optional uint32 random_name_b4 = 16;
+  optional uint32 random_name_b5 = 17;
+  optional uint32 random_name_b6 = 18;
+  optional uint32 random_name_b7 = 19;
+  optional uint32 random_name_b8 = 20;
+  optional uint32 random_name_b9 = 21;
+  optional uint32 random_name_c0 = 22;
+  optional uint32 random_name_c1 = 23;
+  optional uint32 random_name_c2 = 24;
+  optional uint32 random_name_c3 = 25;
+  optional uint32 random_name_c4 = 26;
+  optional uint32 random_name_c5 = 27;
+  optional uint32 random_name_c6 = 28;
+  optional uint32 random_name_c7 = 29;
+  optional uint32 random_name_c8 = 30;
+  optional uint32 random_name_c9 = 31;
+  optional string version = 32;
+}
diff --git a/tests/bindings/lua/test_upb.lua b/tests/bindings/lua/test_upb.lua
index 5f1aae8837..329c78ac6b 100644
--- a/tests/bindings/lua/test_upb.lua
+++ b/tests/bindings/lua/test_upb.lua
@@ -831,6 +831,14 @@ function test_gc()
   end
 end
 
+function test_b9440()
+  local m = upb_test.HelloRequest()
+  m.id = 8
+  assert_equal(8, m.id)
+  m.version = "1"
+  assert_equal(8, m.id)
+end
+
 local stats = lunit.main()
 
 if stats.failed > 0 or stats.errors > 0 then
diff --git a/upb/def.c b/upb/def.c
index d8db187c7e..9fe8c7e9fa 100644
--- a/upb/def.c
+++ b/upb/def.c
@@ -1574,7 +1574,7 @@ static void make_layout(symtab_addctx* ctx, const upb_MessageDef* m) {
   }
 
   /* Account for space used by hasbits. */
-  l->size = div_round_up(hasbit, 8);
+  l->size = hasbit ? div_round_up(hasbit + 1, 8) : 0;
 
   /* Allocate non-oneof fields. */
   for (int i = 0; i < m->field_count; i++) {
diff --git a/upbc/message_layout.cc b/upbc/message_layout.cc
index dd2edcbc0b..be63a105c8 100644
--- a/upbc/message_layout.cc
+++ b/upbc/message_layout.cc
@@ -190,7 +190,7 @@ void MessageLayout::PlaceNonOneofFields(
   }
 
   // Place hasbits at the beginning.
-  hasbit_bytes_ = DivRoundUp(hasbit_count_, 8);
+  hasbit_bytes_ = hasbit_count_ ? DivRoundUp(hasbit_count_ + 1, 8) : 0;
   Place(SizeAndAlign{{hasbit_bytes_, hasbit_bytes_}, {1, 1}});
 
   // Place non-oneof fields.