@@ -17,10 +17,12 @@
 #import "OFObject.h"
 #import "OFLocking.h"
 
 #import "threading.h"
 
+OF_ASSUME_NONNULL_BEGIN
+
 /*!
  * @class OFMutex OFMutex.h ObjFW/OFMutex.h
  *
  * @brief A class for creating mutual exclusions.
  */
@@ -36,5 +38,7 @@
  *
  * @return A new autoreleased mutex.
  */
 + (instancetype)mutex;
 @end
+
+OF_ASSUME_NONNULL_END