diff --git a/source/threads.tex b/source/threads.tex
index 7925987968..ba8ddf5ea8 100644
--- a/source/threads.tex
+++ b/source/threads.tex
@@ -8079,7 +8079,7 @@
 
     // \ref{thread.lock.shared.obs}, observers
     bool owns_lock() const noexcept;
-    explicit operator bool () const noexcept;
+    explicit operator bool() const noexcept;
     mutex_type* mutex() const noexcept;
 
   private: