diff --git a/egui/Cargo.toml b/egui/Cargo.toml
index 0ee258a0..cfcda442 100644
--- a/egui/Cargo.toml
+++ b/egui/Cargo.toml
@@ -47,7 +47,7 @@ cint = ["epaint/cint"]
 
 persistence = ["serde", "epaint/persistence", "ron"]
 
-# Only needed if you plan to use the same egui::Context from multiple threads.
+# multi_threaded is only needed if you plan to use the same egui::Context from multiple threads.
 single_threaded = ["epaint/single_threaded"]
 multi_threaded = ["epaint/multi_threaded"]
 
diff --git a/epaint/src/mutex.rs b/epaint/src/mutex.rs
index ef23337f..dec4d86b 100644
--- a/epaint/src/mutex.rs
+++ b/epaint/src/mutex.rs
@@ -2,6 +2,9 @@
 //!
 //! When the `single_threaded` feature is on the mutexes will panic when locked from different threads.
 
+#[cfg(not(any(feature = "single_threaded", feature = "multi_threaded")))]
+compile_error!("Either feature \"single_threaded\" or \"multi_threaded\" must be enabled.");
+
 // ----------------------------------------------------------------------------
 
 /// The lock you get from [`Mutex`].