diff --git a/src/protocol/bittorrent/common.c b/src/protocol/bittorrent/common.c
index 51213c3d..d9fe3ecb 100644
--- a/src/protocol/bittorrent/common.c
+++ b/src/protocol/bittorrent/common.c
@@ -21,6 +21,8 @@
 #include "util/string.h"
 #include "util/snprintf.h"
 
+const bittorrent_id_T BITTORRENT_NULL_ID;
+
 /* Debug function which returns printable peer ID. */
 unsigned char *
 get_peer_id(bittorrent_id_T peer_id)
diff --git a/src/protocol/bittorrent/common.h b/src/protocol/bittorrent/common.h
index 5206bebf..5ea507e1 100644
--- a/src/protocol/bittorrent/common.h
+++ b/src/protocol/bittorrent/common.h
@@ -45,7 +45,7 @@ struct terminal;
 typedef sha1_digest_bin_T bittorrent_id_T;
 
 /** Special peer ID used for determining whether an ID has been set. */
-const bittorrent_id_T BITTORRENT_NULL_ID;
+extern const bittorrent_id_T BITTORRENT_NULL_ID;
 
 /** Check if the ID has been set. */
 #define bittorrent_id_is_empty(id) \