diff --git a/enum.c b/enum.c
index a49e4b0b74..d378e48967 100644
--- a/enum.c
+++ b/enum.c
@@ -515,7 +515,14 @@ enum_flat_map(VALUE obj)
 static VALUE
 enum_to_a(int argc, VALUE *argv, VALUE obj)
 {
-    VALUE ary = rb_ary_new();
+    VALUE ary, size = rb_check_funcall(obj, id_size, 0, 0);
+
+    if (size == Qundef) {
+	ary = rb_ary_new();
+    }
+    else {
+	ary = rb_ary_new2(NUM2LONG(size));
+    }
 
     rb_block_call(obj, id_each, argc, argv, collect_all, ary);
     OBJ_INFECT(ary, obj);