diff --git a/tests/draft-next/optional/format/duration.json b/tests/draft-next/optional/format/duration.json
index d5adca20..c4aa66ba 100644
--- a/tests/draft-next/optional/format/duration.json
+++ b/tests/draft-next/optional/format/duration.json
@@ -46,6 +46,11 @@
                 "data": "PT1D",
                 "valid": false
             },
+            {
+                "description": "must start with P",
+                "data": "4DT12H30M5S",
+                "valid": false
+            },
             {
                 "description": "no elements present",
                 "data": "P",
diff --git a/tests/draft2019-09/optional/format/duration.json b/tests/draft2019-09/optional/format/duration.json
index 00d5f47a..2d515a64 100644
--- a/tests/draft2019-09/optional/format/duration.json
+++ b/tests/draft2019-09/optional/format/duration.json
@@ -46,6 +46,11 @@
                 "data": "PT1D",
                 "valid": false
             },
+            {
+                "description": "must start with P",
+                "data": "4DT12H30M5S",
+                "valid": false
+            },
             {
                 "description": "no elements present",
                 "data": "P",
diff --git a/tests/draft2020-12/optional/format/duration.json b/tests/draft2020-12/optional/format/duration.json
index a3af56ef..a09fec5e 100644
--- a/tests/draft2020-12/optional/format/duration.json
+++ b/tests/draft2020-12/optional/format/duration.json
@@ -46,6 +46,11 @@
                 "data": "PT1D",
                 "valid": false
             },
+            {
+                "description": "must start with P",
+                "data": "4DT12H30M5S",
+                "valid": false
+            },
             {
                 "description": "no elements present",
                 "data": "P",