diff --git a/packages/runtime/src/scr_async.c b/packages/runtime/src/scr_async.c
index 752854fdf..bf58cdd36 100644
--- a/packages/runtime/src/scr_async.c
+++ b/packages/runtime/src/scr_async.c
@@ -1664,6 +1664,54 @@ static void scr_switch(ScrCtx *from, ScrCtx *to, ScrFiber *to_fiber) {
 #endif
 }
 
+#ifdef SCR_NODE_ISLANDS
+/* A nested entry into Node (scr_node_island.c) runs on Node's stack in a
+ * context of its own while the fiber that made the call stays suspended in
+ * it. Runtime code that Node calls back there runs as the loop's code does:
+ * no current fiber, the main exception cell and ALS slot. It may resume
+ * other fibers, which saves the resumer's context in scr_loop_ctx (and,
+ * under ASan, its fake stack in the main slot); those belong to whoever
+ * resumed the calling fiber, so they are saved here and put back when the
+ * nested entry returns, or the calling fiber's next park would return to a
+ * context that no longer exists. */
+typedef struct {
+  ScrFiber *current;
+  ScrAlsCtx **als;
+  ScrCtx loop_ctx;
+#ifdef SCR_ASAN_FIBERS
+  void *main_fake;
+#endif
+} ScrNestedSave;
+
+void *scr_loop_nested_enter(void) {
+  ScrNestedSave *s = malloc(sizeof *s);
+  if (s == NULL) scr_oom();
+  s->current = scr_current;
+  s->als = scr_als_active;
+  s->loop_ctx = scr_loop_ctx;
+#ifdef SCR_ASAN_FIBERS
+  s->main_fake = scr_main_fake_stack;
+  scr_main_fake_stack = NULL;
+#endif
+  scr_current = NULL;
+  scr_exc_swap_cell(NULL);
+  scr_als_active = &scr_als_main_slot;
+  return s;
+}
+
+void scr_loop_nested_leave(void *save) {
+  ScrNestedSave *s = save;
+  scr_loop_ctx = s->loop_ctx;
+#ifdef SCR_ASAN_FIBERS
+  scr_main_fake_stack = s->main_fake;
+#endif
+  scr_current = s->current;
+  scr_exc_swap_cell(s->current ? &s->current->exc : NULL);
+  scr_als_active = s->als;
+  free(s);
+}
+#endif
+
 static void scr_promise_settle_wake(ScrPromise *p);
 
 /* Copies src's settlement into a still-pending dst (payload RETAINED —
diff --git a/packages/runtime/src/scr_node_island.c b/packages/runtime/src/scr_node_island.c
index 5c4a9b3a3..1effa3e0a 100644
--- a/packages/runtime/src/scr_node_island.c
+++ b/packages/runtime/src/scr_node_island.c
@@ -332,6 +332,7 @@ static void ni_on_node_stack_below_mark(void (*job)(void *), void *arg) {
   ni_nested_return = &caller;
   uintptr_t guard = scr_stack_guard;
   SCR_STACK_GUARD_SET(scr_stack_guard_for((uintptr_t)lo, hi - (uintptr_t)lo));
+  void *loop_save = scr_loop_nested_enter();
 #ifdef NI_ASAN
   void *caller_fake = NULL;
   __sanitizer_start_switch_fiber(&caller_fake, lo, hi - (uintptr_t)lo);
@@ -342,6 +343,7 @@ static void ni_on_node_stack_below_mark(void (*job)(void *), void *arg) {
   size_t size;
   __sanitizer_finish_switch_fiber(caller_fake, &bottom, &size);
 #endif
+  scr_loop_nested_leave(loop_save);
   SCR_STACK_GUARD_SET(guard);
 }
 
diff --git a/packages/runtime/src/scr_node_island.h b/packages/runtime/src/scr_node_island.h
index 2029a5c16..b477ec3b9 100644
--- a/packages/runtime/src/scr_node_island.h
+++ b/packages/runtime/src/scr_node_island.h
@@ -39,6 +39,11 @@ void scr_loop_host(const struct ScrNodeBridge *bridge, double since_ms);
  * fiber; 0: none), and the test for Node's stack it uses. */
 extern uintptr_t scr_node_stack_low;
 extern bool (*scr_node_stack_has)(uintptr_t sp);
+/* Around a nested entry: the runtime's scheduling state of the suspended
+ * calling fiber, saved and put back (the SCR_NODE_ISLANDS build of
+ * scr_async.c). */
+void *scr_loop_nested_enter(void);
+void scr_loop_nested_leave(void *save);
 bool scr_loop_is_hosted(void);
 void scr_loop_hosted_run(uint32_t kind, uint32_t token);
 void scr_loop_hosted_io_state(struct ScrNodeIoState *state);