diff --git a/src/user.zig b/src/user.zig
index 4168e7c..7751854 100644
--- a/src/user.zig
+++ b/src/user.zig
@@ -34,10 +34,10 @@ pub const UserWriter = struct {
     // function makes tests easier, and removes the Shell dependency of this
     // module.
     const shellIndexFnType = fn ([]const u8) ?u6;
-    appendTo: ArrayList(u8),
+    appendTo: *ArrayList(u8),
     shellIndexFn: shellIndexFnType,
 
-    pub fn init(appendTo: ArrayList(u8), shellIndexFn: shellIndexFnType) UserWriter {
+    pub fn init(appendTo: *ArrayList(u8), shellIndexFn: shellIndexFnType) UserWriter {
         return UserWriter{
             .appendTo = appendTo,
             .shellIndexFn = shellIndexFn,
@@ -98,7 +98,7 @@ test "construct PackedUser blob" {
     var buf = ArrayList(u8).init(testing.allocator);
     defer buf.deinit();
 
-    var writer = UserWriter.init(buf, testShellIndex);
+    var writer = UserWriter.init(&buf, testShellIndex);
     const user1 = User{
         .uid = 1000,
         .gid = 1000,