diff --git a/src/address.c b/src/address.c
index 8d7c98da..40d024ad 100644
--- a/src/address.c
+++ b/src/address.c
@@ -68,7 +68,7 @@ int coap_is_mcast(const coap_address_t *a) {
 #else /* !defined(WITH_CONTIKI) && !defined(WITH_LWIP) */
 
 /* make compilers happy that do not like empty modules */
-COAP_STATIC_INLINE void dummy()
+COAP_STATIC_INLINE void dummy(void)
 {
 }
 
diff --git a/src/async.c b/src/async.c
index d8a48f5d..0460f4d3 100644
--- a/src/async.c
+++ b/src/async.c
@@ -106,5 +106,5 @@ coap_free_async(coap_async_state_t *s) {
 }
 
 #else
-void does_not_exist();	/* make some compilers happy */
+void does_not_exist(void);	/* make some compilers happy */
 #endif /* WITHOUT_ASYNC */
diff --git a/src/coap_time.c b/src/coap_time.c
index df8316f5..9231c3bd 100644
--- a/src/coap_time.c
+++ b/src/coap_time.c
@@ -129,7 +129,7 @@ coap_tick_t coap_ticks_from_rt_us(uint64_t t) {
 #else /* HAVE_TIME_H */
 
 /* make compilers happy that do not like empty modules */
-COAP_STATIC_INLINE void dummy()
+COAP_STATIC_INLINE void dummy(void)
 {
 }