Index: src/OFPlugin.h
==================================================================
--- src/OFPlugin.h
+++ src/OFPlugin.h
@@ -55,12 +55,12 @@
 #ifdef __cplusplus
 extern "C" {
 #endif
 extern of_plugin_handle_t of_dlopen(OFString *path, int flags);
 extern void *of_dlsym(of_plugin_handle_t handle, const char *symbol);
-extern OFString *of_dlerror(void);
+extern OFString *_Nullable of_dlerror(void);
 extern void of_dlclose(of_plugin_handle_t handle);
 #ifdef __cplusplus
 }
 #endif
 
 OF_ASSUME_NONNULL_END