diff --git a/lib/minifb/src/minifb.zig b/lib/minifb/src/minifb.zig
index b15956b..b0511d6 100644
--- a/lib/minifb/src/minifb.zig
+++ b/lib/minifb/src/minifb.zig
@@ -1,4 +1,5 @@
 const std = @import("std");
+const assert = std.debug.assert;
 const c = @cImport({
     @cInclude("MiniFB.h");
     @cInclude("MiniFB_enums.h");
@@ -48,7 +49,9 @@ pub fn Window(comptime TUserData: type) type {
             const cTitle = @as([*c]const u8, title);
             const cwin: ?*c.mfb_window = c.mfb_open_ex(cTitle, width, height, intFlags);
             if (cwin) |value| {
-                return Window(TUserData){ .cwin = value };
+                const win = Window(TUserData){ .cwin = value };
+                assert(@bitCast(usize, win) == @ptrToInt(win.cwin));
+                return win;
             } else {
                 return error.ItBroke;
             }
@@ -75,12 +78,10 @@ pub fn Window(comptime TUserData: type) type {
 
         pub fn getUserData(self: Window(TUserData)) ?*TUserData {
             var cData = c.mfb_get_user_data(self.cwin);
-            std.log.info("@alignOf cwin={d}, addr={x}", .{ @alignOf(@TypeOf(self.cwin)), @ptrToInt(self.cwin) });
-            std.log.info("@alignOf cdata={d}, addr={x}", .{ @alignOf(@TypeOf(cData)), @ptrToInt(cData) });
             return @ptrCast(?*TUserData, @alignCast(@alignOf(?*TUserData), cData));
         }
 
-        pub const ActiveFunc = fn (win: Window(TUserData), isActive: bool) void;
+        pub const ActiveFunc = fn (win: Window(TUserData), isActive: bool) callconv(.C) void;
         pub fn setActiveCallback(self: Window(TUserData), callback: ActiveFunc) void {
             c.mfb_set_active_callback(self.cwin, @ptrCast(c.mfb_active_func, callback));
         }
@@ -93,13 +94,13 @@ test "user data is null if never set" {
     try testing.expectEqual(@as(?*u64, null), data);
 }
 
-// test "user data is not null if previously set" {
-//     const win = try Window(u64).open("abc", 100, 100, .{});
-//     var data: u64 = 42;
-//     win.setUserData(&data);
-//     const expected: u64 = 42;
-//     try testing.expectEqual(expected, win.getUserData().?.*);
-// }
+test "user data is not null if previously set" {
+    const win = try Window(u64).open("abc", 100, 100, .{});
+    var data: u64 = 42;
+    win.setUserData(&data);
+    const expected: u64 = 42;
+    try testing.expectEqual(expected, win.getUserData().?.*);
+}
 
 pub fn getTargetFPS() u32 {
     return c.mfb_get_target_fps();
diff --git a/src/main.zig b/src/main.zig
index f8cf967..061fb22 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -98,7 +98,7 @@ const Buffer = struct {
     }
 };
 
-fn handleActive(win: mfb.Window(State), isActive: bool) void {
+fn handleActive(win: mfb.Window(State), isActive: bool) callconv(.C) void {
     // win.getUserData().?.*.active = isActive;
     var data = win.getUserData();
     std.log.info("got data", .{});