diff --git a/py/gc.c b/py/gc.c
index 7ed60fe2cc..99eacfbe6f 100644
--- a/py/gc.c
+++ b/py/gc.c
@@ -95,6 +95,12 @@
 #define BLOCK_FROM_PTR(area, ptr) (((byte *)(ptr) - area->gc_pool_start) / BYTES_PER_BLOCK)
 #define PTR_FROM_BLOCK(area, block) (((block) * BYTES_PER_BLOCK + (uintptr_t)area->gc_pool_start))
 
+// After the ATB, there must be a byte filled with AT_FREE so that gc_mark_tree
+// cannot erroneously conclude that a block extends past the end of the GC heap
+// due to bit patterns in the FTB (or first block, if finalizers are disabled)
+// being interpreted as AT_TAIL.
+#define ALLOC_TABLE_GAP_BYTE (1)
+
 #if MICROPY_ENABLE_FINALISER
 // FTB = finaliser table byte
 // if set, then the corresponding block may have a finaliser
@@ -123,16 +129,16 @@ STATIC void gc_setup_area(mp_state_mem_area_t *area, void *start, void *end) {
     // => T = A * (1 + BLOCKS_PER_ATB / BLOCKS_PER_FTB + BLOCKS_PER_ATB * BYTES_PER_BLOCK)
     size_t total_byte_len = (byte *)end - (byte *)start;
     #if MICROPY_ENABLE_FINALISER
-    area->gc_alloc_table_byte_len = total_byte_len * MP_BITS_PER_BYTE / (MP_BITS_PER_BYTE + MP_BITS_PER_BYTE * BLOCKS_PER_ATB / BLOCKS_PER_FTB + MP_BITS_PER_BYTE * BLOCKS_PER_ATB * BYTES_PER_BLOCK);
+    area->gc_alloc_table_byte_len = (total_byte_len - ALLOC_TABLE_GAP_BYTE) * MP_BITS_PER_BYTE / (MP_BITS_PER_BYTE + MP_BITS_PER_BYTE * BLOCKS_PER_ATB / BLOCKS_PER_FTB + MP_BITS_PER_BYTE * BLOCKS_PER_ATB * BYTES_PER_BLOCK);
     #else
-    area->gc_alloc_table_byte_len = total_byte_len / (1 + MP_BITS_PER_BYTE / 2 * BYTES_PER_BLOCK);
+    area->gc_alloc_table_byte_len = (total_byte_len - ALLOC_TABLE_GAP_BYTE) / (1 + MP_BITS_PER_BYTE / 2 * BYTES_PER_BLOCK);
     #endif
 
     area->gc_alloc_table_start = (byte *)start;
 
     #if MICROPY_ENABLE_FINALISER
     size_t gc_finaliser_table_byte_len = (area->gc_alloc_table_byte_len * BLOCKS_PER_ATB + BLOCKS_PER_FTB - 1) / BLOCKS_PER_FTB;
-    area->gc_finaliser_table_start = area->gc_alloc_table_start + area->gc_alloc_table_byte_len;
+    area->gc_finaliser_table_start = area->gc_alloc_table_start + area->gc_alloc_table_byte_len + ALLOC_TABLE_GAP_BYTE;
     #endif
 
     size_t gc_pool_block_len = area->gc_alloc_table_byte_len * BLOCKS_PER_ATB;
@@ -143,12 +149,12 @@ STATIC void gc_setup_area(mp_state_mem_area_t *area, void *start, void *end) {
     assert(area->gc_pool_start >= area->gc_finaliser_table_start + gc_finaliser_table_byte_len);
     #endif
 
-    // clear ATBs
-    memset(area->gc_alloc_table_start, 0, area->gc_alloc_table_byte_len);
-
     #if MICROPY_ENABLE_FINALISER
-    // clear FTBs
-    memset(area->gc_finaliser_table_start, 0, gc_finaliser_table_byte_len);
+    // clear ATBs and FTBs
+    memset(area->gc_alloc_table_start, 0, gc_finaliser_table_byte_len + area->gc_alloc_table_byte_len + ALLOC_TABLE_GAP_BYTE);
+    #else
+    // clear ATBs
+    memset(area->gc_alloc_table_start, 0, area->gc_alloc_table_byte_len + ALLOC_TABLE_GAP_BYTE);
     #endif
 
     area->gc_last_free_atb_index = 0;