diff --git a/psycopg/typecast_datetime.c b/psycopg/typecast_datetime.c
index f24223cb..e34117dd 100644
--- a/psycopg/typecast_datetime.c
+++ b/psycopg/typecast_datetime.c
@@ -406,6 +406,11 @@ typecast_PYINTERVAL_cast(const char *str, Py_ssize_t len, PyObject *curs)
             }
             break;
 
+        case 'P':
+            PyErr_SetString(NotSupportedError,
+                "iso_8601 intervalstyle currently not supported");
+            return NULL;
+
         default:
             break;
         }
diff --git a/tests/test_dates.py b/tests/test_dates.py
index 74dfc9ab..85216f87 100755
--- a/tests/test_dates.py
+++ b/tests/test_dates.py
@@ -438,6 +438,13 @@ class DatetimeTests(ConnectingTestCase, CommonDatetimeTestsMixin):
             r = cur.fetchone()[0]
             self.assertEqual(r, v, "%s -> %s != %s" % (s, r, v))
 
+    def test_interval_iso_8601_not_supported(self):
+        # We may end up supporting, but no pressure for it
+        cur = self.conn.cursor()
+        cur.execute("set local intervalstyle to iso_8601")
+        cur.execute("select '1 day 2 hours'::interval")
+        self.assertRaises(psycopg2.NotSupportedError, cur.fetchone)
+
 
 # Only run the datetime tests if psycopg was compiled with support.
 if not hasattr(psycopg2.extensions, 'PYDATETIME'):