diff --git a/tests/tdef/tdef_test.c b/tests/tdef/tdef_test.c
index 60066b1..276330a 100644
--- a/tests/tdef/tdef_test.c
+++ b/tests/tdef/tdef_test.c
@@ -179,6 +179,9 @@
 	print_tdef_get_short(tdefs, 7, OSMO_TDEF_M);
 	print_tdef_get_short(tdefs, 7, OSMO_TDEF_CUSTOM);
 
+	printf("setting 23 = 50 (EEXIST)\n");
+	OSMO_ASSERT(osmo_tdef_set(tdefs, 23, 50, OSMO_TDEF_S) == -EEXIST);
+
 	printf("resetting\n");
 	osmo_tdefs_reset(tdefs);
 	print_tdef_info(7);
diff --git a/tests/tdef/tdef_test.ok b/tests/tdef/tdef_test.ok
index 2a3617e..3c4a093 100644
--- a/tests/tdef/tdef_test.ok
+++ b/tests/tdef/tdef_test.ok
@@ -117,6 +117,7 @@
 osmo_tdef_get(7, s)	= 420
 osmo_tdef_get(7, m)	= 7
 osmo_tdef_get(7, custom-unit)	= 420
+setting 23 = 50 (EEXIST)
 resetting
 T7=50s
 osmo_tdef_get(7, s)	= 50