diff --git a/bin/tests/system/checkconf/bad-duration.conf b/bin/tests/system/checkconf/bad-duration.conf
new file mode 100644
index 0000000000..1fbecc7229
--- /dev/null
+++ b/bin/tests/system/checkconf/bad-duration.conf
@@ -0,0 +1,16 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * SPDX-License-Identifier: MPL-2.0
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0.  If a copy of the MPL was not distributed with this
+ * file, you can obtain one at https://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+dnssec-policy "test" {
+	dnskey-ttl xPT1H;
+};
diff --git a/lib/isccfg/duration.c b/lib/isccfg/duration.c
index 30d110ab0a..0a7501b022 100644
--- a/lib/isccfg/duration.c
+++ b/lib/isccfg/duration.c
@@ -61,10 +61,10 @@ isccfg_duration_fromtext(isc_textregion_t *source,
 	duration->unlimited = false;
 
 	/* Every duration starts with 'P' */
-	P = strpbrk(str, "Pp");
-	if (P == NULL) {
+	if (toupper(str[0]) != 'P') {
 		return (ISC_R_BADNUMBER);
 	}
+	P = str;
 
 	/* Record the time indicator. */
 	T = strpbrk(str, "Tt");