diff --git a/scan_engine.cc b/scan_engine.cc
index 1410ebf82..9f4d25945 100644
--- a/scan_engine.cc
+++ b/scan_engine.cc
@@ -821,7 +821,13 @@ void UltraProbe::setConnect(u16 portno) {
 ConnectScanInfo::ConnectScanInfo() {
   maxValidSD = -1;
   numSDs = 0;
-  maxSocketsAllowed = (o.max_parallelism)? o.max_parallelism : MAX(5, max_sd() - 4);
+  /* Subtracting 5 from max_sd accounts for
+     stdin
+     stdout
+     stderr
+     /dev/tty
+     /var/run/utmpx, which is opened on Mac OS X at least. */
+  maxSocketsAllowed = (o.max_parallelism)? o.max_parallelism : MAX(5, max_sd() - 5);
   FD_ZERO(&fds_read);
   FD_ZERO(&fds_write);
   FD_ZERO(&fds_except);