diff --git a/service/service.go b/service/service.go
index bc5037a..0ca7066 100644
--- a/service/service.go
+++ b/service/service.go
@@ -2,6 +2,7 @@ package service
 
 import (
 	"bytes"
+	"fmt"
 	"io"
 	"net"
 	"os/exec"
@@ -104,6 +105,10 @@ func (s *Service) shouldStop() bool {
 }
 
 func (s *Service) proxy(src string, dst string) error {
+	if dst == "" {
+		return fmt.Errorf("destination address is required")
+	}
+
 	ln, err := net.Listen("tcp", src)
 	if err != nil {
 		return err
diff --git a/test/errors.bats b/test/errors.bats
index bfb8088..36184a1 100755
--- a/test/errors.bats
+++ b/test/errors.bats
@@ -6,7 +6,14 @@ load helpers
   "$YNETD" | grep -q 'no services configured'
 }
 
-@test "bad destination address" {
+@test "no address" {
+  ynetd -proxy ":$LISTEN_PORT "
+  knock
+  # Should be instant, rather than timing out.
+  ylog | grep -qF -- "error starting listener: destination address is required"
+}
+
+@test "no port" {
   ynetd -proxy ":$LISTEN_PORT foobar"
   knock
   # Should be instant, rather than timing out.