diff --git a/lib/include/tree_sitter/api.h b/lib/include/tree_sitter/api.h
index c7de7662..0b64e60e 100644
--- a/lib/include/tree_sitter/api.h
+++ b/lib/include/tree_sitter/api.h
@@ -148,10 +148,15 @@ typedef struct {
 /**
  * Switch to a new allocator.
  *
- * Returns the old allocator. The caller needs to take care of any allocated
- * memory managed by the old allocator.
+ * Returns true iff the switch successes.
+ *
+ * This function can only be invoked *once*, before tree-sitter has allocated
+ * any memory via malloc/calloc/realloc. Otherwise, memory bugs could occur
+ * since some memory is allocated by the old allocator, but freed by the new
+ * one.
+ *
  */
-TSAllocator *ts_set_allocator(TSAllocator *new_alloc);
+bool ts_set_allocator(TSAllocator *new_alloc);
 
 /**
  * Create a new parser.
diff --git a/lib/src/alloc.c b/lib/src/alloc.c
index 391430f7..65c72f11 100644
--- a/lib/src/alloc.c
+++ b/lib/src/alloc.c
@@ -70,10 +70,15 @@ TSAllocator *ts_allocator = &ts_default_allocator;
 
 #endif // defined(TREE_SITTER_ALLOCATION_TRACKING)
 
-TSAllocator *ts_set_allocator(TSAllocator *new_alloc)
+bool ts_set_allocator(TSAllocator *new_alloc)
 {
-	TSAllocator *old = ts_allocator;
-	ts_allocator = new_alloc;
-	return old;
+  static bool using_default_allocator = true;
+  if (!using_default_allocator) {
+    fprintf(stderr, "tree-sitter's allocator can only be set once!");
+    return false;
+  }
+  using_default_allocator = false;
+  ts_allocator = new_alloc;
+  return true;
 }