diff --git a/sys/sys/exterrvar.h b/sys/sys/exterrvar.h
--- a/sys/sys/exterrvar.h
+++ b/sys/sys/exterrvar.h
@@ -27,6 +27,8 @@
 
 #ifdef _KERNEL
 
+struct thread;
+
 #ifndef EXTERR_CATEGORY
 #error "Specify error category before including sys/exterrvar.h"
 #endif