@@ -18,14 +18,15 @@
 #import "OFMutableString.h"
 #import "OFUTF8String.h"
 
 OF_ASSUME_NONNULL_BEGIN
 
+OF_SUBCLASSING_RESTRICTED
 @interface OFMutableUTF8String: OFMutableString
 {
 @public
 	struct of_string_utf8_ivars *restrict _s;
 	struct of_string_utf8_ivars _storage;
 }
 @end
 
 OF_ASSUME_NONNULL_END