diff --git a/include/vm/heap.h b/include/vm/heap.h
index 9eed1a3..6da37b7 100644
--- a/include/vm/heap.h
+++ b/include/vm/heap.h
@@ -78,15 +78,6 @@ int32_t heap_alloc_string(struct string *s);
 
 void heap_describe_slot(int slot);
 
-/*
- * Guarantee `headroom` free slots are available on the heap.
- *
- * XXX: this is a hack to fix an issue with HLL calls where a pointer into the
- *      heap can be made invalid by a heap reallocation triggered within the
- *      body of the HLL function.
- */
-void heap_guarantee(unsigned headroom);
-
 #ifdef VM_PRIVATE
 
 extern uint32_t heap_next_seq;
diff --git a/src/ffi.c b/src/ffi.c
index 5215b4b..28a2a59 100644
--- a/src/ffi.c
+++ b/src/ffi.c
@@ -303,13 +303,13 @@ void hll_call(int libno, int fno)
 	if (!fun->fun)
 		VM_ERROR("Unimplemented HLL function: %s.%s", ain->libraries[libno].name, f->name);
 
-	// XXX: Try to prevent the heap from being reallocated mid-call.
-	//      This only works to the extent that HLL functions can guarantee
-	//      no more than 64 heap allocations occur within the call...
-	heap_guarantee(64);
-
 	void *args[HLL_MAX_ARGS];
 	void *ptrs[HLL_MAX_ARGS];
+	// Copy reference arguments to the stack to protect against heap
+	// reallocation during HLL calls.
+	void *heap_ptrs[HLL_MAX_ARGS];
+	int heap_slots[HLL_MAX_ARGS];
+
 	for (int i = f->nr_arguments - 1; i >= 0; i--) {
 		switch (f->arguments[i].type.data) {
 		case AIN_REF_INT:
@@ -330,7 +330,9 @@ void hll_call(int libno, int fno)
 			break;
 		case AIN_REF_STRING:
 			stack_ptr--;
-			ptrs[i] = &heap[stack[stack_ptr].i].s;
+			heap_slots[i] = stack[stack_ptr].i;
+			heap_ptrs[i] = heap[stack[stack_ptr].i].s;
+			ptrs[i] = &heap_ptrs[i];
 			args[i] = &ptrs[i];
 			break;
 		case AIN_STRUCT:
@@ -341,7 +343,9 @@ void hll_call(int libno, int fno)
 		case AIN_REF_STRUCT:
 		case AIN_REF_ARRAY_TYPE:
 			stack_ptr--;
-			ptrs[i] = &heap[stack[stack_ptr].i].page;
+			heap_slots[i] = stack[stack_ptr].i;
+			heap_ptrs[i] = heap[stack[stack_ptr].i].page;
+			ptrs[i] = &heap_ptrs[i];
 			args[i] = &ptrs[i];
 			break;
 		default:
@@ -370,9 +374,13 @@ void hll_call(int libno, int fno)
 			j++;
 			break;
 		case AIN_REF_STRING:
+			heap[heap_slots[i]].s = heap_ptrs[i];
+			break;
 		case AIN_REF_STRUCT:
-		case AIN_REF_FUNC_TYPE:
 		case AIN_REF_ARRAY_TYPE:
+			heap[heap_slots[i]].page = heap_ptrs[i];
+			break;
+		case AIN_REF_FUNC_TYPE:
 			break;
 		case AIN_ARRAY_TYPE:
 			// Sys41VM doesn't make a copy when passing an array by value.
diff --git a/src/heap.c b/src/heap.c
index 5f371ab..99ebb4d 100644
--- a/src/heap.c
+++ b/src/heap.c
@@ -60,19 +60,6 @@ void heap_grow(size_t new_size)
 	heap_size = new_size;
 }
 
-void heap_guarantee(unsigned headroom)
-{
-	if (heap_size - heap_free_ptr >= headroom)
-		return;
-
-	size_t new_size = heap_size;
-	while (new_size - heap_free_ptr < headroom) {
-		new_size += HEAP_ALLOC_STEP;
-	}
-
-	heap_grow(new_size);
-}
-
 void heap_init(void)
 {
 	if (!heap) {