diff --git a/src/iosource/FD_Set.h b/src/iosource/FD_Set.h
index 402d28b8cb..61e3e7a59b 100644
--- a/src/iosource/FD_Set.h
+++ b/src/iosource/FD_Set.h
@@ -77,6 +77,23 @@ public:
 		return false;
 		}
 
+	/**
+	 * @return whether any file descriptors have been added to the set.
+	 */
+	bool Empty() const
+		{
+		return fds.empty();
+		}
+
+	/**
+	 * @return the greatest file descriptor of all that have been added to the
+	 * set, or -1 if the set is empty.
+	 */
+	int Max() const
+		{
+		return max;
+		}
+
 private:
 	int max;
 	std::set<int> fds;
diff --git a/src/iosource/Manager.cc b/src/iosource/Manager.cc
index 26a48c2911..5a087bc4ef 100644
--- a/src/iosource/Manager.cc
+++ b/src/iosource/Manager.cc
@@ -117,6 +117,9 @@ IOSource* Manager::FindSoonest(double* ts)
 
 		src->Clear();
 		src->src->GetFds(&src->fd_read, &src->fd_write, &src->fd_except);
+		if ( src->fd_read.Empty() ) src->fd_read.Insert(0);
+		if ( src->fd_write.Empty() ) src->fd_write.Insert(0);
+		if ( src->fd_except.Empty() ) src->fd_except.Insert(0);
 		src->SetFds(&fd_read, &fd_write, &fd_except, &maxx);
 		}