diff --git a/examples/natmod/btree/btree_c.c b/examples/natmod/btree/btree_c.c
index 5e8a34ac40..e342b7d903 100644
--- a/examples/natmod/btree/btree_c.c
+++ b/examples/natmod/btree/btree_c.c
@@ -94,7 +94,7 @@ int mp_stream_posix_fsync(void *stream) {
     return res;
 }
 
-mp_obj_type_t btree_type;
+mp_obj_full_type_t btree_type;
 
 #include "extmod/modbtree.c"
 
diff --git a/examples/natmod/framebuf/framebuf.c b/examples/natmod/framebuf/framebuf.c
index 2eff61c817..98b44987b3 100644
--- a/examples/natmod/framebuf/framebuf.c
+++ b/examples/natmod/framebuf/framebuf.c
@@ -8,7 +8,7 @@ void *memset(void *s, int c, size_t n) {
 }
 #endif
 
-mp_obj_type_t mp_type_framebuf;
+mp_obj_full_type_t mp_type_framebuf;
 
 #include "extmod/modframebuf.c"
 
diff --git a/examples/natmod/ure/ure.c b/examples/natmod/ure/ure.c
index 175b93e395..d9e11760a7 100644
--- a/examples/natmod/ure/ure.c
+++ b/examples/natmod/ure/ure.c
@@ -32,8 +32,8 @@ void *memmove(void *dest, const void *src, size_t n) {
     return mp_fun_table.memmove_(dest, src, n);
 }
 
-mp_obj_type_t match_type;
-mp_obj_type_t re_type;
+mp_obj_full_type_t match_type;
+mp_obj_full_type_t re_type;
 
 #include "extmod/modure.c"
 
diff --git a/examples/natmod/uzlib/uzlib.c b/examples/natmod/uzlib/uzlib.c
index 99b3691761..24ab15d6f1 100644
--- a/examples/natmod/uzlib/uzlib.c
+++ b/examples/natmod/uzlib/uzlib.c
@@ -8,7 +8,7 @@ void *memset(void *s, int c, size_t n) {
 }
 #endif
 
-mp_obj_type_t decompio_type;
+mp_obj_full_type_t decompio_type;
 
 #include "extmod/moduzlib.c"
 
diff --git a/extmod/modbtree.c b/extmod/modbtree.c
index 15cb634163..60c6885e6b 100644
--- a/extmod/modbtree.c
+++ b/extmod/modbtree.c
@@ -67,7 +67,7 @@ void __dbpanic(DB *db) {
 }
 
 STATIC mp_obj_btree_t *btree_new(DB *db, mp_obj_t stream) {
-    mp_obj_btree_t *o = mp_obj_malloc(mp_obj_btree_t, &btree_type);
+    mp_obj_btree_t *o = mp_obj_malloc(mp_obj_btree_t, (mp_obj_type_t *)&btree_type);
     o->stream = stream;
     o->db = db;
     o->start_key = mp_const_none;
diff --git a/extmod/modframebuf.c b/extmod/modframebuf.c
index f29eab272f..5347be5643 100644
--- a/extmod/modframebuf.c
+++ b/extmod/modframebuf.c
@@ -841,7 +841,7 @@ STATIC MP_DEFINE_CONST_OBJ_TYPE(
 
 // this factory function is provided for backwards compatibility with old FrameBuffer1 class
 STATIC mp_obj_t legacy_framebuffer1(size_t n_args, const mp_obj_t *args_in) {
-    mp_obj_framebuf_t *o = mp_obj_malloc(mp_obj_framebuf_t, &mp_type_framebuf);
+    mp_obj_framebuf_t *o = mp_obj_malloc(mp_obj_framebuf_t, (mp_obj_type_t *)&mp_type_framebuf);
 
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args_in[0], &bufinfo, MP_BUFFER_WRITE);
diff --git a/extmod/modnetwork.h b/extmod/modnetwork.h
index 3481cc6dcc..55ee4eb4d3 100644
--- a/extmod/modnetwork.h
+++ b/extmod/modnetwork.h
@@ -62,7 +62,8 @@ mp_obj_t mod_network_nic_ifconfig(struct netif *netif, size_t n_args, const mp_o
 struct _mod_network_socket_obj_t;
 
 typedef struct _mod_network_nic_type_t {
-    mp_obj_type_t base;
+    // Ensure that this struct is big enough to hold any type size.
+    mp_obj_full_type_t base;
 
     // API for non-socket operations
     int (*gethostbyname)(mp_obj_t nic, const char *name, mp_uint_t len, uint8_t *ip_out);
diff --git a/extmod/modure.c b/extmod/modure.c
index 4395992405..c0114c14fd 100644
--- a/extmod/modure.c
+++ b/extmod/modure.c
@@ -198,7 +198,7 @@ STATIC void re_print(const mp_print_t *print, mp_obj_t self_in, mp_print_kind_t
 STATIC mp_obj_t ure_exec(bool is_anchored, uint n_args, const mp_obj_t *args) {
     (void)n_args;
     mp_obj_re_t *self;
-    if (mp_obj_is_type(args[0], &re_type)) {
+    if (mp_obj_is_type(args[0], (mp_obj_type_t *)&re_type)) {
         self = MP_OBJ_TO_PTR(args[0]);
     } else {
         self = MP_OBJ_TO_PTR(mod_re_compile(1, args));
@@ -217,7 +217,7 @@ STATIC mp_obj_t ure_exec(bool is_anchored, uint n_args, const mp_obj_t *args) {
         return mp_const_none;
     }
 
-    match->base.type = &match_type;
+    match->base.type = (mp_obj_type_t *)&match_type;
     match->num_matches = caps_num / 2; // caps_num counts start and end pointers
     match->str = args[1];
     return MP_OBJ_FROM_PTR(match);
@@ -282,7 +282,7 @@ MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(re_split_obj, 2, 3, re_split);
 
 STATIC mp_obj_t re_sub_helper(size_t n_args, const mp_obj_t *args) {
     mp_obj_re_t *self;
-    if (mp_obj_is_type(args[0], &re_type)) {
+    if (mp_obj_is_type(args[0], (mp_obj_type_t *)&re_type)) {
         self = MP_OBJ_TO_PTR(args[0]);
     } else {
         self = MP_OBJ_TO_PTR(mod_re_compile(1, args));
@@ -305,7 +305,7 @@ STATIC mp_obj_t re_sub_helper(size_t n_args, const mp_obj_t *args) {
     vstr_t vstr_return;
     vstr_return.buf = NULL; // We'll init the vstr after the first match
     mp_obj_match_t *match = mp_local_alloc(sizeof(mp_obj_match_t) + caps_num * sizeof(char *));
-    match->base.type = &match_type;
+    match->base.type = (mp_obj_type_t *)&match_type;
     match->num_matches = caps_num / 2; // caps_num counts start and end pointers
     match->str = where;
 
@@ -430,7 +430,7 @@ STATIC mp_obj_t mod_re_compile(size_t n_args, const mp_obj_t *args) {
     if (size == -1) {
         goto error;
     }
-    mp_obj_re_t *o = mp_obj_malloc_var(mp_obj_re_t, char, size, &re_type);
+    mp_obj_re_t *o = mp_obj_malloc_var(mp_obj_re_t, char, size, (mp_obj_type_t *)&re_type);
     #if MICROPY_PY_URE_DEBUG
     int flags = 0;
     if (n_args > 1) {
diff --git a/extmod/network_ninaw10.c b/extmod/network_ninaw10.c
index 0906176d20..806819648d 100644
--- a/extmod/network_ninaw10.c
+++ b/extmod/network_ninaw10.c
@@ -774,7 +774,7 @@ static const mp_rom_map_elem_t nina_locals_dict_table[] = {
 
 static MP_DEFINE_CONST_DICT(nina_locals_dict, nina_locals_dict_table);
 
-STATIC MP_DEFINE_CONST_OBJ_TYPE(
+STATIC MP_DEFINE_CONST_OBJ_FULL_TYPE(
     mod_network_nic_type_nina_base,
     MP_QSTR_nina,
     MP_TYPE_FLAG_NONE,
diff --git a/extmod/network_wiznet5k.c b/extmod/network_wiznet5k.c
index 951a2966c9..1d7318827c 100644
--- a/extmod/network_wiznet5k.c
+++ b/extmod/network_wiznet5k.c
@@ -1024,7 +1024,7 @@ MP_DEFINE_CONST_OBJ_TYPE(
     locals_dict, &wiznet5k_locals_dict
     );
 #else // WIZNET5K_PROVIDED_STACK
-STATIC MP_DEFINE_CONST_OBJ_TYPE(
+STATIC MP_DEFINE_CONST_OBJ_FULL_TYPE(
     mod_network_nic_type_wiznet5k_base,
     MP_QSTR_WIZNET5K,
     MP_TYPE_FLAG_NONE,
diff --git a/ports/cc3200/mods/modnetwork.h b/ports/cc3200/mods/modnetwork.h
index 6ec90a2bac..c15a6467f2 100644
--- a/ports/cc3200/mods/modnetwork.h
+++ b/ports/cc3200/mods/modnetwork.h
@@ -36,7 +36,8 @@
  DEFINE TYPES
  ******************************************************************************/
 typedef struct _mod_network_nic_type_t {
-    mp_obj_type_t base;
+    // Ensure that this struct is big enough to hold any type size.
+    mp_obj_full_type_t base;
 } mod_network_nic_type_t;
 
 typedef struct _mod_network_socket_base_t {
diff --git a/ports/cc3200/mods/modwlan.c b/ports/cc3200/mods/modwlan.c
index 1c99f075e9..1e82b07e08 100644
--- a/ports/cc3200/mods/modwlan.c
+++ b/ports/cc3200/mods/modwlan.c
@@ -1285,7 +1285,7 @@ STATIC const mp_rom_map_elem_t wlan_locals_dict_table[] = {
 };
 STATIC MP_DEFINE_CONST_DICT(wlan_locals_dict, wlan_locals_dict_table);
 
-STATIC MP_DEFINE_CONST_OBJ_TYPE(
+STATIC MP_DEFINE_CONST_OBJ_FULL_TYPE(
     mod_network_nic_type_wlan_base,
     MP_QSTR_WLAN,
     MP_TYPE_FLAG_NONE,
diff --git a/py/obj.h b/py/obj.h
index 4ab7e0dc09..9c0f41dc3d 100644
--- a/py/obj.h
+++ b/py/obj.h
@@ -562,6 +562,9 @@ typedef mp_int_t (*mp_buffer_fun_t)(mp_obj_t obj, mp_buffer_info_t *bufinfo, mp_
 bool mp_get_buffer(mp_obj_t obj, mp_buffer_info_t *bufinfo, mp_uint_t flags);
 void mp_get_buffer_raise(mp_obj_t obj, mp_buffer_info_t *bufinfo, mp_uint_t flags);
 
+// This struct will be updated to become a variable sized struct. In order to
+// use this as a member, or allocate dynamically, use the mp_obj_empty_type_t
+// or mp_obj_full_type_t structs below (which must be kept in sync).
 struct _mp_obj_type_t {
     // A type is an object so must start with this entry, which points to mp_type_type.
     mp_obj_base_t base;
@@ -632,6 +635,13 @@ struct _mp_obj_type_t {
     struct _mp_obj_dict_t *locals_dict;
 };
 
+// Non-variable sized versions of mp_obj_type_t to be used as a member
+// in other structs or for dynamic allocation. The fields are exactly
+// as in mp_obj_type_t, but with a fixed size for the flexible array
+// members.
+typedef mp_obj_type_t mp_obj_empty_type_t;
+typedef mp_obj_type_t mp_obj_full_type_t;
+
 #define MP_TYPE_NULL_MAKE_NEW (NULL)
 
 // Implementation of MP_DEFINE_CONST_OBJ_TYPE for each number of arguments.
@@ -657,14 +667,15 @@ struct _mp_obj_type_t {
 // of the 30th argument (30 is 13*2 + 4).
 #define MP_DEFINE_CONST_OBJ_TYPE_NARGS(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, _18, _19, _20, _21, _22, _23, _24, _25, _26, _27, _28, _29, N, ...) MP_DEFINE_CONST_OBJ_TYPE_NARGS_##N
 
-// This macro is used to define a object type in ROM.
+// These macros are used to define a object type in ROM.
 // Invoke as MP_DEFINE_CONST_OBJ_TYPE(_typename, _name, _flags, _make_new [, slot, func]*)
-// It uses the number of arguments to select which MP_DEFINE_CONST_OBJ_TYPE_*
+// They use the number of arguments to select which MP_DEFINE_CONST_OBJ_TYPE_*
 // macro to use based on the number of arguments. It works by shifting the
 // numeric values 12, 11, ... 0 by the number of arguments, such that the
 // 30th argument ends up being the number to use. The _INV values are
 // placeholders because the slot arguments come in pairs.
 #define MP_DEFINE_CONST_OBJ_TYPE(...) MP_DEFINE_CONST_OBJ_TYPE_EXPAND(MP_DEFINE_CONST_OBJ_TYPE_NARGS(__VA_ARGS__, _INV, 12, _INV, 11, _INV, 10, _INV, 9, _INV, 8, _INV, 7, _INV, 6, _INV, 5, _INV, 4, _INV, 3, _INV, 2, _INV, 1, _INV, 0)(mp_obj_type_t, __VA_ARGS__))
+#define MP_DEFINE_CONST_OBJ_FULL_TYPE(...) MP_DEFINE_CONST_OBJ_TYPE_EXPAND(MP_DEFINE_CONST_OBJ_TYPE_NARGS(__VA_ARGS__, _INV, 12, _INV, 11, _INV, 10, _INV, 9, _INV, 8, _INV, 7, _INV, 6, _INV, 5, _INV, 4, _INV, 3, _INV, 2, _INV, 1, _INV, 0)(mp_obj_full_type_t, __VA_ARGS__))
 
 // Constant types, globally accessible
 extern const mp_obj_type_t mp_type_type;
diff --git a/py/objnamedtuple.h b/py/objnamedtuple.h
index d32af35afe..9f23351d5a 100644
--- a/py/objnamedtuple.h
+++ b/py/objnamedtuple.h
@@ -29,7 +29,10 @@
 #include "py/objtuple.h"
 
 typedef struct _mp_obj_namedtuple_type_t {
-    mp_obj_type_t base;
+    // Must use the full-size version to avoid this being a variable sized member.
+    // This means that named tuples use slightly more RAM than necessary, but
+    // no worse than if we didn't have slots/split representation.
+    mp_obj_full_type_t base;
     size_t n_fields;
     qstr fields[];
 } mp_obj_namedtuple_type_t;
diff --git a/py/objtype.c b/py/objtype.c
index 77fe8e22e3..cc5a5e5802 100644
--- a/py/objtype.c
+++ b/py/objtype.c
@@ -1148,7 +1148,10 @@ mp_obj_t mp_obj_new_type(qstr name, mp_obj_t bases_tuple, mp_obj_t locals_dict)
         #endif
     }
 
-    mp_obj_type_t *o = m_new0(mp_obj_type_t, 1);
+    // Allocate a full-sized mp_obj_full_type_t instance (i.e. all slots / extended fields).
+    // Given that Python types use almost all the slots anyway, this doesn't cost anything
+    // extra.
+    mp_obj_type_t *o = (mp_obj_type_t *)m_new0(mp_obj_full_type_t, 1);
     o->base.type = &mp_type_type;
     o->flags = base_flags;
     o->name = name;