diff --git a/apps/er-coap-03/er-coap-03-observing.h b/apps/er-coap-03/er-coap-03-observing.h
index cde1974ae..b9baad377 100644
--- a/apps/er-coap-03/er-coap-03-observing.h
+++ b/apps/er-coap-03/er-coap-03-observing.h
@@ -39,6 +39,7 @@
 #ifndef COAP_OBSERVING_H_
 #define COAP_OBSERVING_H_
 
+#include "sys/stimer.h"
 #include "er-coap-03.h"
 #include "er-coap-03-transactions.h"
 
diff --git a/apps/er-coap-07/er-coap-07-observing.h b/apps/er-coap-07/er-coap-07-observing.h
index 62f9f7df4..2fecdaf02 100644
--- a/apps/er-coap-07/er-coap-07-observing.h
+++ b/apps/er-coap-07/er-coap-07-observing.h
@@ -39,6 +39,7 @@
 #ifndef COAP_OBSERVING_H_
 #define COAP_OBSERVING_H_
 
+#include "sys/stimer.h"
 #include "er-coap-07.h"
 #include "er-coap-07-transactions.h"
 
diff --git a/apps/er-coap-12/er-coap-12-observing.h b/apps/er-coap-12/er-coap-12-observing.h
index 349a0fc4f..78aa64d05 100644
--- a/apps/er-coap-12/er-coap-12-observing.h
+++ b/apps/er-coap-12/er-coap-12-observing.h
@@ -39,6 +39,7 @@
 #ifndef COAP_OBSERVING_H_
 #define COAP_OBSERVING_H_
 
+#include "sys/stimer.h"
 #include "er-coap-12.h"
 #include "er-coap-12-transactions.h"