diff --git a/crates/egui/src/lib.rs b/crates/egui/src/lib.rs
index bc90f0cf9..d86851a1d 100644
--- a/crates/egui/src/lib.rs
+++ b/crates/egui/src/lib.rs
@@ -685,7 +685,7 @@ pub fn __run_test_ctx(mut run_ui: impl FnMut(&Context)) {
 }
 
 /// For use in tests; especially doctests.
-pub fn __run_test_ui(add_contents: impl Fn(&mut Ui)) {
+pub fn __run_test_ui(mut add_contents: impl FnMut(&mut Ui)) {
     let ctx = Context::default();
     ctx.set_fonts(FontDefinitions::empty()); // prevent fonts from being loaded (save CPU time)
     let _ = ctx.run_ui(Default::default(), |ui| {