diff --git a/src/etc/inc/interfaces.inc b/src/etc/inc/interfaces.inc
index b3fc36987..91c3271ea 100644
--- a/src/etc/inc/interfaces.inc
+++ b/src/etc/inc/interfaces.inc
@@ -2937,8 +2937,8 @@ function interface_dhcpv6_prepare($interface = 'wan', $wancfg)
         if (is_numeric($wancfg['dhcp6-ia-pd-len'])) {
             /* Setup the prefix delegation */
             $dhcp6cconf .= "id-assoc pd 0 {\n";
-            $preflen = 64 - $wancfg['dhcp6-ia-pd-len'];
             if (isset($wancfg['dhcp6-ia-pd-send-hint'])) {
+                $preflen = 64 - $wancfg['dhcp6-ia-pd-len'];
                 $dhcp6cconf .= "  prefix ::/{$preflen} infinity;\n";
             }
             $iflist = link_interface_to_track6($interface);
diff --git a/src/etc/inc/services.inc b/src/etc/inc/services.inc
index 8e2d9f6d1..c312cf758 100644
--- a/src/etc/inc/services.inc
+++ b/src/etc/inc/services.inc
@@ -323,6 +323,7 @@ function services_radvd_configure($blacklist = array())
         $ifcfgipv6 = get_interface_ipv6($if);
 
         if ($autotype == 'slaac') {
+            /* XXX this may be incorrect and needs an override or revisit */
             $subnetv6 = '2000::';
         } elseif (is_ipaddrv6($ifcfgipv6)) {
             $ifcfgsnv6 = get_interface_subnetv6($if);
@@ -347,7 +348,8 @@ function services_radvd_configure($blacklist = array())
         $radvdconf .= sprintf("\tAdvLinkMTU %s;\n", !empty($mtu) ? $mtu : 1280);
         $radvdconf .= "\tAdvOtherConfigFlag on;\n";
         $radvdconf .= "\tprefix {$subnetv6}/{$ifcfgsnv6} {\n";
-        if ($autotype == 'slaac') {
+        /* if delegation is off or unavailable use the tracked interface prefix */
+        if (calculate_ipv6_delegation_length($trackif) < 0) {
             $radvdconf .= "\t\tBase6Interface $realtrackif;\n";
             $radvdconf .= "\t\tDeprecatePrefix on;\n";
         }
@@ -1112,16 +1114,16 @@ function services_dhcpdv6_configure($blacklist = array(), $verbose = false)
             continue;
         }
         if (!empty($config['interfaces'][$ifname]['track6-interface'])) {
-            $realif = get_real_interface($ifname, "inet6");
+            $realif = get_real_interface($ifname, 'inet6');
             $ifcfgipv6 = get_interface_ipv6($ifname);
             if (!is_ipaddrv6($ifcfgipv6)) {
                 continue;
             }
+
             $ifcfgipv6 = Net_IPv6::getNetmask($ifcfgipv6, 64);
-            $trackifname = $config['interfaces'][$ifname]['track6-interface'];
-            $trackcfg = $config['interfaces'][$trackifname];
-            $pdlen = calculate_ipv6_delegation_length($trackifname);
+
             if (!isset($config['interfaces'][$ifname]['dhcpd6track6allowoverride'])) {
+                /* mock a real server */
                 $dhcpdv6cfg[$ifname] = array();
                 $dhcpdv6cfg[$ifname]['enable'] = true;
 
@@ -1133,7 +1135,8 @@ function services_dhcpdv6_configure($blacklist = array(), $verbose = false)
                 $ifcfgipv6arr[7] = '2000';
                 $dhcpdv6cfg[$ifname]['range']['to'] = Net_IPv6::compress(implode(':', $ifcfgipv6arr));
 
-                /* prefix length > 0? We can add dhcp6 prefix delegation server */
+                /* with enough room we can add dhcp6 prefix delegation */
+                $pdlen = calculate_ipv6_delegation_length($config['interfaces'][$ifname]['track6-interface']);
                 if ($pdlen > 2) {
                     $pdlenmax = $pdlen;
                     $pdlenhalf = $pdlenmax - 1;
diff --git a/src/etc/inc/util.inc b/src/etc/inc/util.inc
index 7c1d66b52..d48d4fc87 100644
--- a/src/etc/inc/util.inc
+++ b/src/etc/inc/util.inc
@@ -231,36 +231,40 @@ function gen_subnetv6_max($ipaddr, $bits)
     return $result;
 }
 
-/* Returns the calculated bit length of the prefix delegation from the WAN interface */
-/* DHCP-PD is variable, calculate from the prefix-len on the WAN interface */
-/* 6rd is variable, calculate from 64 - (v6 prefixlen - (32 - v4 prefixlen)) */
-/* 6to4 is 16 bits, e.g. 65535 */
-function calculate_ipv6_delegation_length($if) {
+/* returns the calculated bit length of the prefix delegation from a WAN interface */
+function calculate_ipv6_delegation_length($if)
+{
     global $config;
 
-    if(!isset($config['interfaces'][$if]) || !is_array($config['interfaces'][$if])) {
-        return false;
-    } elseif (!isset($config['interfaces'][$if]['ipaddrv6'])) {
-        return 0;
+    $pdlen = -1;
+
+    if (!isset($config['interfaces'][$if]['ipaddrv6'])) {
+        return $pdlen;
     }
 
-    switch($config['interfaces'][$if]['ipaddrv6']) {
-        case "6to4":
+    switch ($config['interfaces'][$if]['ipaddrv6']) {
+        case '6to4':
             $pdlen = 16;
             break;
-        case "6rd":
+        case '6rd':
             $rd6cfg = $config['interfaces'][$if];
-            $rd6plen = explode("/", $rd6cfg['prefix-6rd']);
-            $pdlen = (64 - ($rd6plen[1] + (32 - $rd6cfg['prefix-6rd-v4plen'])));
+            $rd6plen = explode('/', $rd6cfg['prefix-6rd']);
+            $pdlen = 64 - ($rd6plen[1] + (32 - $rd6cfg['prefix-6rd-v4plen']));
+            if ($pdlen == 0) {
+                /* XXX bug reports this is not working, needs investigation */
+                $pdlen = -1;
+            }
             break;
-        case "dhcp6":
+        case 'dhcp6':
             $dhcp6cfg = $config['interfaces'][$if];
-            $pdlen = $dhcp6cfg['dhcp6-ia-pd-len'];
+            if (is_numeric($dhcp6cfg['dhcp6-ia-pd-len'])) {
+                $pdlen = $dhcp6cfg['dhcp6-ia-pd-len'];
+            }
             break;
         default:
-            $pdlen = 0;
             break;
     }
+
     return $pdlen;
 }