diff --git a/src/udp_engine.cpp b/src/udp_engine.cpp
index 9cff8162..f18d0b63 100644
--- a/src/udp_engine.cpp
+++ b/src/udp_engine.cpp
@@ -48,7 +48,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 zmq::udp_engine_t::udp_engine_t() :
     plugged (false),
-    fd(NULL),
+    fd(-1),
     session(NULL),
     handle(NULL),
     address(nullptr),