diff --git a/libco.h b/libco.h
index 88d00a72..633cd542 100644
--- a/libco.h
+++ b/libco.h
@@ -13,12 +13,12 @@ extern "C" {
 
 typedef void* cothread_t;
 
-cothread_t co_active();
+cothread_t co_active(void);
 cothread_t co_derive(void*, unsigned int, void (*)(void));
 cothread_t co_create(unsigned int, void (*)(void));
 void co_delete(cothread_t);
 void co_switch(cothread_t);
-int co_serializable();
+int co_serializable(void);
 
 #ifdef __cplusplus
 }