diff --git a/lua/def.c b/lua/def.c
index 90161ec819..a7a26c89c0 100644
--- a/lua/def.c
+++ b/lua/def.c
@@ -617,7 +617,11 @@ const upb_FileDef* lupb_FileDef_check(lua_State* L, int narg) {
 
 static int lupb_FileDef_Dependency(lua_State* L) {
   const upb_FileDef* f = lupb_FileDef_check(L, 1);
+#if defined(LUA_VERSION_NUM) && LUA_VERSION_NUM >= 501
+  int index = luaL_checkinteger(L, 2);
+#else
   int index = luaL_checkint(L, 2);
+#endif
   const upb_FileDef* dep = upb_FileDef_Dependency(f, index);
   lupb_wrapper_pushwrapper(L, 1, dep, LUPB_FILEDEF);
   return 1;
@@ -631,7 +635,11 @@ static int lupb_FileDef_DependencyCount(lua_State* L) {
 
 static int lupb_FileDef_enum(lua_State* L) {
   const upb_FileDef* f = lupb_FileDef_check(L, 1);
+#if defined(LUA_VERSION_NUM) && LUA_VERSION_NUM >= 501
+  int index = luaL_checkinteger(L, 2);
+#else
   int index = luaL_checkint(L, 2);
+#endif
   const upb_EnumDef* e = upb_FileDef_TopLevelEnum(f, index);
   lupb_wrapper_pushwrapper(L, 1, e, LUPB_ENUMDEF);
   return 1;
@@ -645,7 +653,11 @@ static int lupb_FileDef_enumcount(lua_State* L) {
 
 static int lupb_FileDef_msg(lua_State* L) {
   const upb_FileDef* f = lupb_FileDef_check(L, 1);
+#if defined(LUA_VERSION_NUM) && LUA_VERSION_NUM >= 501
+  int index = luaL_checkinteger(L, 2);
+#else
   int index = luaL_checkint(L, 2);
+#endif
   const upb_MessageDef* m = upb_FileDef_TopLevelMessage(f, index);
   lupb_wrapper_pushwrapper(L, 1, m, LUPB_MSGDEF);
   return 1;
diff --git a/lua/upb.c b/lua/upb.c
index 8c73e5fb7f..f6ff01114d 100644
--- a/lua/upb.c
+++ b/lua/upb.c
@@ -61,11 +61,13 @@
 /* Lua compatibility code *****************************************************/
 
 /* Shims for upcoming Lua 5.3 functionality. */
+#if LUA_VERSION_NUM < 503
 static bool lua_isinteger(lua_State* L, int argn) {
   LUPB_UNUSED(L);
   LUPB_UNUSED(argn);
   return false;
 }
+#endif
 
 /* Utility functions **********************************************************/
 
diff --git a/lua/upbc.cc b/lua/upbc.cc
index e857f7aae9..8efb491d0e 100644
--- a/lua/upbc.cc
+++ b/lua/upbc.cc
@@ -79,7 +79,7 @@ static void PrintString(int max_cols, absl::string_view* str,
     } else if (ch == '\'') {
       printer->PrintRaw("\\'");
       max_cols--;
-    } else if (isprint(ch)) {
+    } else if (isprint(static_cast<int>(static_cast<unsigned char>(ch)))) {
       printer->WriteRaw(&ch, 1);
       max_cols--;
     } else {
diff --git a/upb/text/encode.c b/upb/text/encode.c
index c694088fc2..33438be30b 100644
--- a/upb/text/encode.c
+++ b/upb/text/encode.c
@@ -308,7 +308,7 @@ static void txtenc_map(txtenc* e, const upb_Map* map, const upb_FieldDef* f) {
 #define CHK(x)      \
   do {              \
     if (!(x)) {     \
-      return false; \
+      return NULL; \
     }               \
   } while (0)