diff --git a/apps/er-coap/er-coap-separate.h b/apps/er-coap/er-coap-separate.h
index dbecf1b39..33a923240 100644
--- a/apps/er-coap/er-coap-separate.h
+++ b/apps/er-coap/er-coap-separate.h
@@ -61,7 +61,7 @@ typedef struct coap_separate {
 
 int coap_separate_handler(resource_t *resource, void *request,
                           void *response);
-void coap_separate_reject();
+void coap_separate_reject(void);
 void coap_separate_accept(void *request, coap_separate_t *separate_store);
 void coap_separate_resume(void *response, coap_separate_t *separate_store,
                           uint8_t code);