diff --git a/src/gc.c b/src/gc.c
index 4937c65ddf84e..6ff2b3e6a8851 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -1238,30 +1238,51 @@ NOINLINE static int gc_mark_module(jl_ptls_t ptls, jl_module_t *m, int d)
     return refyoung;
 }
 
+// Handle the case where the stack is only partially copied.
+STATIC_INLINE uintptr_t gc_get_stack_addr(void *_addr, uintptr_t offset,
+                                          uintptr_t lb, uintptr_t ub)
+{
+    uintptr_t addr = (uintptr_t)_addr;
+    if (addr >= lb && addr < ub)
+        return addr + offset;
+    return addr;
+}
+
+STATIC_INLINE uintptr_t gc_read_stack(void *_addr, uintptr_t offset,
+                                      uintptr_t lb, uintptr_t ub)
+{
+    uintptr_t real_addr = gc_get_stack_addr(_addr, offset, lb, ub);
+    return *(uintptr_t*)real_addr;
+}
+
 static void gc_mark_stack(jl_ptls_t ptls, jl_value_t *ta, jl_gcframe_t *s,
-                          intptr_t offset, int d)
+                          uintptr_t offset, uintptr_t lb, uintptr_t ub, int d)
 {
     while (s != NULL) {
-        s = (jl_gcframe_t*)((char*)s + offset);
-        jl_value_t ***rts = (jl_value_t***)(((void**)s)+2);
-        size_t nr = s->nroots>>1;
-        if (s->nroots & 1) {
-            for(size_t i=0; i < nr; i++) {
-                jl_value_t **ptr = (jl_value_t**)((char*)rts[i] + offset);
-                if (*ptr != NULL) {
-                    gc_push_root(ptls, *ptr, d);
+        jl_value_t ***rts = (jl_value_t***)(((void**)s) + 2);
+        size_t nroots = gc_read_stack(&s->nroots, offset, lb, ub);
+        size_t nr = nroots >> 1;
+        if (nroots & 1) {
+            for (size_t i = 0; i < nr; i++) {
+                void **slot = (void**)gc_read_stack(&rts[i], offset, lb, ub);
+                void *obj = (void*)gc_read_stack(slot, offset, lb, ub);
+                if (obj != NULL) {
+                    gc_push_root(ptls, obj, d);
                 }
             }
         }
         else {
-            for(size_t i=0; i < nr; i++) {
-                if (rts[i] != NULL) {
-                    verify_parent2("task", ta, &rts[i], "stack(%d)", (int)i);
-                    gc_push_root(ptls, rts[i], d);
+            for (size_t i=0; i < nr; i++) {
+                void *obj = (void*)gc_read_stack(&rts[i], offset, lb, ub);
+                if (obj) {
+                    verify_parent2("task", ta,
+                                   gc_get_stack_addr(&rts[i], offset, lb, ub),
+                                   "stack(%d)", (int)i);
+                    gc_push_root(ptls, obj, d);
                 }
             }
         }
-        s = s->prev;
+        s = (jl_gcframe_t*)gc_read_stack(&s->prev, offset, lb, ub);
     }
 }
 
@@ -1284,16 +1305,19 @@ static void gc_mark_task_stack(jl_ptls_t ptls, jl_task_t *ta, int d)
 #endif
     }
     if (ta == ptls2->current_task) {
-        gc_mark_stack(ptls, (jl_value_t*)ta, ptls2->pgcstack, 0, d);
+        gc_mark_stack(ptls, (jl_value_t*)ta, ptls2->pgcstack,
+                      0, 0, (uintptr_t)-1, d);
     }
     else if (stkbuf) {
-        intptr_t offset;
+        uintptr_t offset = 0;
+        uintptr_t lb = 0;
+        uintptr_t ub = (uintptr_t)-1;
 #ifdef COPY_STACKS
-        offset = (char *)ta->stkbuf - ((char *)ptls2->stackbase - ta->ssize);
-#else
-        offset = 0;
+        ub = (uintptr_t)ptls2->stackbase;
+        lb = ub - ta->ssize;
+        offset = (uintptr_t)ta->stkbuf - lb;
 #endif
-        gc_mark_stack(ptls, (jl_value_t*)ta, ta->gcstack, offset, d);
+        gc_mark_stack(ptls, (jl_value_t*)ta, ta->gcstack, offset, lb, ub, d);
     }
 }
 
diff --git a/src/task.c b/src/task.c
index 22cbef056857d..da456112945a8 100644
--- a/src/task.c
+++ b/src/task.c
@@ -147,7 +147,8 @@ static void NOINLINE save_stack(jl_ptls_t ptls, jl_task_t *t)
     if (t->state == done_sym || t->state == failed_sym)
         return;
     char *frame_addr = (char*)jl_get_frame_addr();
-    size_t nb = (char*)ptls->stackbase - frame_addr;
+    char *stackbase = (char*)ptls->stackbase;
+    size_t nb = stackbase > frame_addr ? stackbase - frame_addr : 0;
     char *buf;
     if (t->stkbuf == NULL || t->bufsz < nb) {
         buf = (char*)jl_gc_alloc_buf(ptls, nb);