diff --git a/ports/zephyr-cp/supervisor/port.c b/ports/zephyr-cp/supervisor/port.c
index 1ecde0b0141..a46ce7c68f6 100644
--- a/ports/zephyr-cp/supervisor/port.c
+++ b/ports/zephyr-cp/supervisor/port.c
@@ -43,6 +43,18 @@
 static tlsf_t heap;
 static size_t tlsf_heap_used = 0;
 
+// TLSF writes its control structure into the first pool it is given, but
+// dimensions that structure from the maximum heap size rather than from the
+// region itself: tlsf_create_with_pool() forwards max_bytes to tlsf_create(),
+// so both of its size checks test the maximum, not the space available. A
+// region too small for the structure is therefore overrun rather than
+// rejected. On a SiWx917-DK2605A with an 8 MB maximum the structure is 2412
+// bytes and landed in a 1 KB region, overrunning it by 1388 bytes.
+//
+// Only the region hosting the control structure has to satisfy this; smaller
+// regions are still fine as additional pools.
+#define MIN_FIRST_POOL_SIZE (8 * 1024)
+
 // Auto generated in pins.c
 extern const struct device *const rams[];
 extern const uint32_t *const ram_bounds[];
@@ -290,10 +302,23 @@ void port_heap_init(void) {
         }
         #endif
 
+        if (valid_pool_count == 0 && size < MIN_FIRST_POOL_SIZE) {
+            // Too small to hold the control structure. Leave it for a later
+            // pass rather than letting TLSF overrun it.
+            printk("Skipping region at %p: too small to host the heap control structure\n", heap_bottom);
+            pools[i] = NULL;
+            continue;
+        }
+
         printk("Init heap at %p - %p with size %d\n", heap_bottom, heap_top, size);
         // If this crashes, then make sure you've enabled all of the Kconfig needed for the drivers.
         if (valid_pool_count == 0) {
             heap = tlsf_create_with_pool(heap_bottom, size, circuitpy_max_ram_size);
+            if (heap == NULL) {
+                printk("Heap creation failed at %p; trying the next region\n", heap_bottom);
+                pools[i] = NULL;
+                continue;
+            }
             pools[i] = tlsf_get_pool(heap);
         } else {
             pools[i] = tlsf_add_pool(heap, heap_bottom + 1, size - sizeof(uint32_t));