--- a/sys/dev/raid/mps/mps_mapping.c
+++ b/sys/dev/raid/mps/mps_mapping.c
@@ -232,6 +232,12 @@
 	} else
 		*start_idx = sc->max_devices - sc->max_volumes;
 	*end_idx = *start_idx + sc->max_volumes - 1;
+	/* Keep the IR range inside the mapping table: with firmware facts
+	 * MaxTargets == 0 plus RESERVED_TARGETID_0 and low volume mapping,
+	 * start_idx + max_volumes - 1 == max_devices, and the `<=` IR walks
+	 * would touch mapping_table[max_devices] (one entry OOB). */
+	if (*end_idx >= sc->max_devices)
+		*end_idx = sc->max_devices - 1;
 }
 
 /**