diff --git a/limmutable.c b/limmutable.c
index 22a7cb4..c58bf9b 100644
--- a/limmutable.c
+++ b/limmutable.c
@@ -38,16 +38,21 @@ static int SHARED_CLASS_METATABLE;
 
 static int Lcreate_instance(lua_State *L)
 {
+    size_t n_args = lua_gettop(L)-1;
     // arg 1: class table, ...
     lua_getfield(L, 1, "__fields");
     size_t n = lua_objlen(L, -1);
+    if (n_args > n) {
+        luaL_error(L, "Too many args: expected %d, but got %d", n, n_args);
+    }
     lua_pop(L, 1);
     
     // Compute the hash:
     lua_Integer hash = (lua_Integer)lua_topointer(L, 1); // Hash depends on the metatable used in creation
     for (lua_Integer i=1; i <=(lua_Integer)n; i++) {
         lua_Integer item_hash;
-        switch (lua_type(L, 1+i)) {
+        int type = n > n_args ? LUA_TNIL : lua_type(L, 1+i);
+        switch (type) {
             case LUA_TNIL: case LUA_TNONE:
                 // Arbitrarily chosen value
                 item_hash = 0x97167da9;
@@ -55,8 +60,8 @@ static int Lcreate_instance(lua_State *L)
             case LUA_TNUMBER:
                 {
                     // Cast float bits to integer
-                    lua_Number n = lua_tonumber(L, 1+i);
-                    item_hash = *((lua_Integer*)&n);
+                    lua_Number num = lua_tonumber(L, 1+i);
+                    item_hash = *((lua_Integer*)&num);
                     break;
                 }
             case LUA_TBOOLEAN:
@@ -98,7 +103,7 @@ static int Lcreate_instance(lua_State *L)
         // Stack: [buckets, nil]
         lua_pop(L, 1);
         // Stack: [buckets]
-        lua_createtable(L, 1, 0);
+        lua_createtable(L, 0, 1);
         // Stack: [buckets, bucket]
         lua_pushlightuserdata(L, (void*)&SHARED_BUCKET_METATABLE);
         lua_gettable(L, LUA_REGISTRYINDEX);
@@ -116,7 +121,7 @@ static int Lcreate_instance(lua_State *L)
     while (lua_next(L, -2) != 0) { // for hash_collider_inst, hash_collider in pairs(bucket) do
         // Stack: [buckets, bucket, hash_collider_inst, hash_collider]
         int bucket_item_matches = 1;
-        // Perform a full equality check
+        // Shallow equality check:
         lua_pushnil(L);
         while (lua_next(L, -2) != 0) { // for i, collider_value in pairs(hash_collider) do
             // Stack: [buckets, bucket, hash_collider_inst, hash_collider, i, value]
@@ -135,7 +140,7 @@ static int Lcreate_instance(lua_State *L)
         if (bucket_item_matches) {
             // Stack: [buckets, bucket, hash_collider_inst, hash_collider]
             lua_pop(L, 1);
-            // Found matching singleton
+            // Found matching pre-existing instance
             return 1;
         }
     }
@@ -154,14 +159,21 @@ static int Lcreate_instance(lua_State *L)
     // Stack [buckets, bucket, inst_userdata, inst_userdata]
     lua_createtable(L, n, 0); // Create the table to store the instance's data
     // Stack [buckets, bucket, inst_userdata, inst_userdata, inst_table]
-    for (lua_Integer i=1; i <=(lua_Integer)n; i++) {
+    lua_Integer i;
+    for (i=1; i <= (lua_Integer)n_args; i++) {
         lua_pushvalue(L, i+1);
         // Stack [buckets, bucket, inst_userdata, inst_userdata, inst_table, arg #1+i]
         lua_rawseti(L, -2, i);
     }
+    for (; i <= (lua_Integer)n; i++) {
+        lua_pushnil(L);
+        // Stack [buckets, bucket, inst_userdata, inst_userdata, inst_table, nil]
+        lua_rawseti(L, -2, i);
+    }
     // Stack [buckets, bucket, inst_userdata, inst_userdata, inst_table]
     lua_settable(L, -4); // buckets[inst_userdata] = inst_table
     // Stack [buckets, bucket, inst_userdata]
+    lua_gc(L, LUA_GCSTEP, 1);
     return 1;
 }
 
@@ -499,8 +511,7 @@ static const luaL_Reg Rinstance_metamethods[] =
 static int Lmake_class(lua_State *L)
 {
     // immutable([fields], [methods/metamethods])
-    
-    lua_newtable(L);
+    lua_createtable(L, 0, 16); // Rough guess, 16 fields from Rinstance_metamethods + __fields, etc.
     // Stack: [CLS]
     // Populate CLS.__len, CLS.__index, CLS.__pairs, etc.
     luaL_register(L,NULL,Rinstance_metamethods);
@@ -525,7 +536,7 @@ static int Lmake_class(lua_State *L)
     }
 
     // Stack: [CLS]
-    lua_newtable(L);
+    lua_createtable(L, 0, 32); // Rough guess: at least 32 instances concurrently
     // Stack: [CLS, CLS.buckets]
     lua_setfield(L, -2, "__instances");