diff --git a/transporthandler.cpp b/transporthandler.cpp
index 74d1817..fbd127f 100644
--- a/transporthandler.cpp
+++ b/transporthandler.cpp
@@ -1041,7 +1041,9 @@
         log<level::ERR>("Invalid netmask", entry("NETMASK=%s", maskStr));
         elog<InternalFailure>();
     }
-    return 32 - __builtin_ctz(x);
+    return static_cast<bool>(x)
+               ? AddrFamily<AF_INET>::defaultPrefix - __builtin_ctz(x)
+               : 0;
 }
 
 // We need to store this value so it can be returned to the client