diff --git a/src/zeek.bif b/src/zeek.bif
index a69764cf7e..4f991ef37e 100644
--- a/src/zeek.bif
+++ b/src/zeek.bif
@@ -2877,7 +2877,7 @@ function raw_bytes_to_v6_addr%(x: string%): addr
 function to_port%(s: string%): port
     %{
     int port = 0;
-    if ( s->Len() < 10 )
+    if ( s->Len() > 0 && s->Len() < 10 )
         {
         char* slash;
         errno = 0;
diff --git a/testing/btest/Baseline/bifs.to_port/out b/testing/btest/Baseline/bifs.to_port/out
index ab28024817..dd88826ec0 100644
--- a/testing/btest/Baseline/bifs.to_port/out
+++ b/testing/btest/Baseline/bifs.to_port/out
@@ -6,6 +6,9 @@
 0/udp
 0/icmp
 0/unknown
+0/unknown
+123/unknown
+123/unknown
 256/tcp
 256/udp
 256/icmp
diff --git a/testing/btest/bifs/to_port.zeek b/testing/btest/bifs/to_port.zeek
index b1e220f982..8588dc229e 100644
--- a/testing/btest/bifs/to_port.zeek
+++ b/testing/btest/bifs/to_port.zeek
@@ -11,6 +11,9 @@ event zeek_init()
 	print to_port("0/udp");
 	print to_port("0/icmp");
 	print to_port("not a port");
+	print to_port("");
+	print to_port("123");
+	print to_port("123/");
 
 	local a: transport_proto = tcp;
 	local b: transport_proto = udp;