diff --git a/Development/nmos/node_interfaces.cpp b/Development/nmos/node_interfaces.cpp
index 2af5e40d..55a56dda 100644
--- a/Development/nmos/node_interfaces.cpp
+++ b/Development/nmos/node_interfaces.cpp
@@ -20,13 +20,35 @@ namespace nmos
             return chassis_id.is_null() ? utility::string_t{} : chassis_id.as_string();
         }
 
+        bool is_valid_node_interfaces_port_id(const utility::string_t& port_id)
+        {
+            if (17 != port_id.size()) return false;
+
+            for (size_t index = 0; index < port_id.size(); ++index)
+            {
+                const auto character = port_id[index];
+                if (2 == index % 3)
+                {
+                    if (U('-') != character) return false;
+                }
+                else if (!((U('0') <= character && character <= U('9')) || (U('a') <= character && character <= U('f'))))
+                {
+                    return false;
+                }
+            }
+
+            return true;
+        }
+
         // Port ID must be a MAC address
         web::json::value make_node_interfaces_port_id(const utility::string_t& port_id)
         {
             using web::json::value;
-            // when no physical address is available, use the common null value of all zeros
+            // IS-04 port_id requires the six-octet lowercase-hyphen form. Any other representation,
+            // including uppercase or colon-separated MAC addresses, is not representable in this field
+            // and uses the existing null-address fallback of all zeros
             // see https://standards.ieee.org/content/dam/ieee-standards/standards/web/documents/tutorials/eui.pdf
-            return value::string(!port_id.empty() ? port_id : U("00-00-00-00-00-00"));
+            return value::string(is_valid_node_interfaces_port_id(port_id) ? port_id : U("00-00-00-00-00-00"));
         }
     }
 
diff --git a/Development/nmos/test/node_interfaces_test.cpp b/Development/nmos/test/node_interfaces_test.cpp
index 29bbae62..4f540298 100644
--- a/Development/nmos/test/node_interfaces_test.cpp
+++ b/Development/nmos/test/node_interfaces_test.cpp
@@ -4,6 +4,22 @@
 #include "bst/test/test.h"
 #include "nmos/json_fields.h"
 
+namespace
+{
+    utility::string_t make_node_interface_port_id(const utility::string_t& port_id)
+    {
+        const nmos::node_interface iface{
+            U("aa-bb-cc-dd-ee-01"),
+            port_id,
+            U("tunl0"),
+            U(""),
+            U("")
+        };
+        auto json = nmos::make_node_interface(iface);
+        return nmos::fields::port_id(json);
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////////////////
 BST_TEST_CASE(testMakeParseNodeInterface)
 {
@@ -45,6 +61,36 @@ BST_TEST_CASE(testMakeParseNodeInterfaceNullChassisId)
     BST_REQUIRE(iface == nmos::parse_node_interface(json));
 }
 
+////////////////////////////////////////////////////////////////////////////////////////////
+BST_TEST_CASE(testMakeNodeInterfaceValidPortIdUnchanged)
+{
+    BST_REQUIRE_EQUAL(U("aa-bb-cc-dd-ee-ff"), make_node_interface_port_id(U("aa-bb-cc-dd-ee-ff")));
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////
+BST_TEST_CASE(testMakeNodeInterfaceEmptyPortIdFallback)
+{
+    BST_REQUIRE_EQUAL(U("00-00-00-00-00-00"), make_node_interface_port_id(U("")));
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////
+BST_TEST_CASE(testMakeNodeInterfaceInvalidPortIdFallback)
+{
+    const std::vector<utility::string_t> invalid_port_ids{
+        U("00-00-00-00"),
+        U("00-00-00-00-00"),
+        U("00-00-00-00-00-00-00"),
+        U("gg-00-00-00-00-00"),
+        U("AA-BB-CC-DD-EE-FF"),
+        U("00:00:00:00:00:00")
+    };
+
+    for (const auto& invalid_port_id : invalid_port_ids)
+    {
+        BST_CHECK_EQUAL(U("00-00-00-00-00-00"), make_node_interface_port_id(invalid_port_id));
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////////////////
 BST_TEST_CASE(testMakeParseNodeInterfaceAttachedNetworkDevice)
 {