diff --git a/doc/doxygen/lwip.Doxyfile b/doc/doxygen/lwip.Doxyfile
index a32c3278..5fe97184 100644
--- a/doc/doxygen/lwip.Doxyfile
+++ b/doc/doxygen/lwip.Doxyfile
@@ -2068,7 +2068,8 @@ INCLUDE_FILE_PATTERNS  = *.c *.h
 # recursively expanded use the := operator instead of the = operator.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-PREDEFINED             = NO_SYS=0 \
+PREDEFINED             = __DOXYGEN__=1 \
+                         NO_SYS=0 \
                          SYS_LIGHTWEIGHT_PROT=1 \
                          LWIP_IPV4=1 \
                          LWIP_IPV6=1 \
diff --git a/src/core/init.c b/src/core/init.c
index 740d2770..092cd2bc 100644
--- a/src/core/init.c
+++ b/src/core/init.c
@@ -36,7 +36,7 @@
  */
 
 /**
- * @defgroup lwip_nosys NO_SYS ("mainloop") mode
+ * @defgroup lwip_nosys Mainloop mode ("NO_SYS")
  * @ingroup lwip
  * Use this mode if you do not run an OS on your system. \#define NO_SYS to 1.
  * Feed incoming packets to netif->input(pbuf, netif) function from mainloop,
diff --git a/src/core/timeouts.c b/src/core/timeouts.c
index 6e16e949..e630b7e2 100644
--- a/src/core/timeouts.c
+++ b/src/core/timeouts.c
@@ -300,7 +300,7 @@ sys_untimeout(sys_timeout_handler handler, void *arg)
  *
  * Must be called periodically from your main loop.
  */
-#if !NO_SYS
+#if !NO_SYS && !defined __DOXYGEN__
 static
 #endif /* !NO_SYS */
 void