diff --git a/src/pkg/net/ip.go b/src/pkg/net/ip.go
index fd6a7d4ee8..0582009b8b 100644
--- a/src/pkg/net/ip.go
+++ b/src/pkg/net/ip.go
@@ -623,6 +623,9 @@ func parseIPv6(s string, zoneAllowed bool) (ip IP, zone string) {
 		for k := ellipsis + n - 1; k >= ellipsis; k-- {
 			ip[k] = 0
 		}
+	} else if ellipsis >= 0 {
+		// Ellipsis must represent at least one 0 group.
+		return nil, zone
 	}
 	return ip, zone
 }
diff --git a/src/pkg/net/ip_test.go b/src/pkg/net/ip_test.go
index 26b53729b8..ffeb9d315e 100644
--- a/src/pkg/net/ip_test.go
+++ b/src/pkg/net/ip_test.go
@@ -25,6 +25,7 @@ var parseIPTests = []struct {
 	{"fe80::1%lo0", nil},
 	{"fe80::1%911", nil},
 	{"", nil},
+	{"a1:a2:a3:a4::b1:b2:b3:b4", nil}, // Issue 6628
 }
 
 func TestParseIP(t *testing.T) {