diff --git a/luprex/cpp/core/luastack.cpp b/luprex/cpp/core/luastack.cpp
index 210f7af6..a0142a0d 100644
--- a/luprex/cpp/core/luastack.cpp
+++ b/luprex/cpp/core/luastack.cpp
@@ -10,6 +10,10 @@ LuaSpecial LuaRegistry(LUA_REGISTRYINDEX);
 LuaNilMarker LuaNil;
 LuaNewTableMarker LuaNewTable;
 
+inline bool ascii_islower(char c) { return (c >= 'a') && (c <= 'z'); }
+inline bool ascii_isupper(char c) { return (c >= 'A') && (c <= 'Z'); }
+inline bool ascii_isdigit(char c) { return (c >= '0') && (c <= '9'); }
+
 LuaFunctionReg::LuaFunctionReg(const char *n, const char *a, const char *d, bool s, lua_CFunction f) {
     name_ = n;
     args_ = a;
@@ -220,12 +224,30 @@ lua_State *LuaCoreStack::newthread(LuaSlot target) const {
     return result;
 }
 
-bool LuaCoreStack::validclassname(std::string_view cname) {
-    if (cname.empty()) return false;
-    if (cname == "_G") return false;
+bool LuaCoreStack::valididentifier(std::string_view str) {
+    if (str.size() == 0) return false;
+    char c=str[0];
+    if ((!ascii_islower(c)) && (!ascii_isupper(c)) && (c!='_')) return false;
+    for (int i = 1; i < int(str.size()); i++) {
+        char c = str[i];
+        if ((!ascii_islower(c)) && (!ascii_isupper(c)) && (!ascii_isdigit(c)) && (c!='_')) return false;
+    }
     return true;
 }
 
+bool LuaCoreStack::validint64(int64_t value) {
+    return (value <= MAXINT) && (value >= -MAXINT);
+}
+
+bool LuaCoreStack::validpositiveint64(int64_t value) {
+    return (value <= MAXINT) && (value >= 1);
+}
+
+bool LuaCoreStack::validclassname(std::string_view cname) {
+    if (cname == "_G") return false;
+    return valididentifier(cname);
+}
+
 bool LuaCoreStack::validclassname(LuaSlot slot) const {
     if (!isstring(slot)) return false;
     return validclassname(ckstring(slot));
@@ -352,6 +374,8 @@ void LuaCoreStack::makeclass(LuaSlot tab, std::string_view name) const {
 }
 
 void LuaCoreStack::maketan(LuaSlot tab, int64_t id) const {
+    assert(validpositiveint64(id));
+
     LuaVar tangibles, metatab;
     LuaExtStack LS(L_, tangibles, metatab);
     
@@ -412,6 +436,20 @@ int64_t LuaCoreStack::tanid(LuaSlot tab) const {
     return result;
 }
 
+bool LuaCoreStack::tangetclass(LuaSlot classobj, LuaSlot tab) {
+    if (istable(tab) && (gettabletype(tab) == LUA_TT_TANGIBLE) && lua_getmetatable(L_, tab.index())) {
+        lua_pushstring(L_, "__index");
+        lua_rawget(L_, -2);
+        lua_replace(L_, classobj);
+        lua_pop(L_, 1);
+        if (istable(classobj) && (gettabletype(classobj) == LUA_TT_CLASS)) {
+            return true;
+        }
+    }
+    set(classobj, LuaNil);
+    return false;
+}
+
 bool LuaCoreStack::issortablekey(LuaSlot s) const {
     int type = lua_type(L_, s);
     return (type == LUA_TBOOLEAN) || (type == LUA_TNUMBER) || (type == LUA_TSTRING);
diff --git a/luprex/cpp/core/luastack.hpp b/luprex/cpp/core/luastack.hpp
index 89fe48b9..1e6d29c5 100644
--- a/luprex/cpp/core/luastack.hpp
+++ b/luprex/cpp/core/luastack.hpp
@@ -417,6 +417,15 @@ public:
 
     int next(LuaSlot tab, LuaSlot key, LuaSlot value) const;
 
+    // Return true if the string is a valid lua identifier.
+    static bool valididentifier(std::string_view id);
+
+    // Return true if the int64 can be stored losslessly in a lua_Number.
+    static bool validint64(int64_t value);
+
+    // Return true if the int64 is storable in lua and is positive.
+    static bool validpositiveint64(int64_t value);
+
     // Return true if the classname is legal.
     bool validclassname(LuaSlot value) const;
     static bool validclassname(std::string_view cname);
@@ -446,6 +455,9 @@ public:
     // stub into a full blown tangible, and World::tangible_delete to turn
     // a full-blown tangible back into a stub.  A stub doesn't have a
     // class or a thread table.
+    //
+    // Assert-fails if the tangible ID is not a validpositiveint64.
+    //
     void maketan(LuaSlot tab, int64_t id) const;
 
     // Return true if a tangible is empty (deleted or not yet created).
@@ -454,6 +466,9 @@ public:
     // Get the ID of a tangible.
     int64_t tanid(LuaSlot tab) const;
 
+    // Get the class of a tangible.
+    bool tangetclass(LuaSlot classobj, LuaSlot tan);
+
     // Return true if the value is a sortable key (string, number, or boolean).
     bool issortablekey(LuaSlot s) const;
 
diff --git a/luprex/cpp/core/serializelua.cpp b/luprex/cpp/core/serializelua.cpp
index 4f5a61cf..116af233 100644
--- a/luprex/cpp/core/serializelua.cpp
+++ b/luprex/cpp/core/serializelua.cpp
@@ -80,11 +80,19 @@ class Deserializer {
                 return;
             }
             case LUA_TT_TANGIBLE: {
-                LS_.maketan(target, sb_->read_int64());
+                int64_t tanid = sb_->read_int64();
+                if (!LS_.validpositiveint64(tanid)) {
+                    throw DeserializeError();
+                }
+                LS_.maketan(target, tanid);
                 return;
             }
             case LUA_TT_CLASS: {
-                LS_.makeclass(target, sb_->read_string());
+                eng::string name = sb_->read_string();
+                if (!LS_.validclassname(name)) {
+                    throw DeserializeError();
+                }
+                LS_.makeclass(target, name);
                 return;
             }
             case LUA_PK_REFERENCE: {
diff --git a/luprex/cpp/core/source.cpp b/luprex/cpp/core/source.cpp
index 636f6b2d..16b6a45d 100644
--- a/luprex/cpp/core/source.cpp
+++ b/luprex/cpp/core/source.cpp
@@ -21,9 +21,6 @@ LuaDefine(makeclass, "classname", "create a class if it doesn't already exist")
     LuaArg classname;
     LuaRet classtab;
     LuaDefStack LS(L, classname, classtab);
-    if (!LS.isstring(classname)) {
-        luaL_error(L, "class name must be a string");
-    }
     if (!LS.validclassname(classname)) {
         luaL_error(L, "invalid class name: %s", LS.ckstring(classname).c_str());
     };
diff --git a/luprex/cpp/core/util.cpp b/luprex/cpp/core/util.cpp
index 7213ca1e..34ea008f 100644
--- a/luprex/cpp/core/util.cpp
+++ b/luprex/cpp/core/util.cpp
@@ -178,14 +178,7 @@ int common_prefix_length(string_view a, string_view b) {
 }
 
 bool is_lua_id(string_view str) {
-    if (str.size() == 0) return false;
-    char c=str[0];
-    if ((!ascii_isalpha(c)) && (c!='_')) return false;
-    for (int i = 1; i < int(str.size()); i++) {
-        char c = str[i];
-        if ((!ascii_isalpha(c)) && (!ascii_isdigit(c)) && (c!='_')) return false;
-    }
-    return true;
+    return LuaCoreStack::valididentifier(str);
 }
 
 bool is_lua_comment(string_view s) {
diff --git a/luprex/cpp/core/world-accessor.cpp b/luprex/cpp/core/world-accessor.cpp
index c43b4d52..b144461e 100644
--- a/luprex/cpp/core/world-accessor.cpp
+++ b/luprex/cpp/core/world-accessor.cpp
@@ -227,13 +227,12 @@ LuaDefine(tangible_getclass, "tan",
     "|The return value is a string, the class name, not"
     "|the class table.") {
     LuaArg tanobj;
-    LuaVar mt, classtab;
+    LuaVar classtab;
     LuaRet classname;
-    LuaDefStack LS(L, tanobj, mt, classtab, classname);
+    LuaDefStack LS(L, tanobj, classtab, classname);
     World *w = World::fetch_global_pointer(L);
     w->tangible_get(LS, tanobj, false);
-    LS.getmetatable(mt, tanobj);
-    LS.rawget(classtab, mt, "__index");
+    LS.tangetclass(classtab, tanobj);
     eng::string name = LS.classname(classtab);
     if (name == "") {
         LS.set(classname, LuaNil);
diff --git a/luprex/cpp/core/world-core.cpp b/luprex/cpp/core/world-core.cpp
index 50960a71..b40fe682 100644
--- a/luprex/cpp/core/world-core.cpp
+++ b/luprex/cpp/core/world-core.cpp
@@ -141,7 +141,7 @@ Tangible *World::tangible_get(const LuaCoreStack &LS, LuaSlot tab, bool allowdel
 }
 
 Tangible *World::tangible_make(const LuaCoreStack &LS0, LuaSlot database, int64_t id) {
-    assert(id != 0);
+    assert(LS0.validpositiveint64(id));
     LuaVar metatab;
     LuaExtStack LS(LS0.state(), metatab);
 
diff --git a/luprex/cpp/core/world-testing.cpp b/luprex/cpp/core/world-testing.cpp
index e9a5dc16..c5304bbd 100644
--- a/luprex/cpp/core/world-testing.cpp
+++ b/luprex/cpp/core/world-testing.cpp
@@ -58,11 +58,17 @@ eng::string World::tangible_ids_debug_string() const {
 }
 
 eng::string World::tangibles_near_debug_string(int64_t actor, int64_t distance) {
+    assert(stack_is_clear());
+    lua_State *L = state();
+    LuaVar tangibles, database, mt;
+    LuaExtStack LS(L, tangibles, database);
+    LS.rawget(tangibles, LuaRegistry, "tangibles");
     eng::ostringstream result;
     util::IdVector tans;
     get_near(actor, distance, true, false, true, &tans);
     for (int64_t id : tans) {
         const Tangible *tan = tangible_get(id);
+        LS.rawget(database, tangibles, id);
         AnimState state = tan->anim_queue_.get_final_everything();
         result << id << ": " << state.debug_string() << std::endl;
     }