diff --git a/arch/arm/core/cortex_m/mpu/arm_mpu.c b/arch/arm/core/cortex_m/mpu/arm_mpu.c
index 1568fcc..dda4466 100644
--- a/arch/arm/core/cortex_m/mpu/arm_mpu.c
+++ b/arch/arm/core/cortex_m/mpu/arm_mpu.c
@@ -108,10 +108,12 @@
 {
 	SYS_LOG_DBG("Region info: 0x%x 0x%x", base, size);
 	/*
-	 * The new MPU regions are are allocated per type after the statically
-	 * configured regions.
+	 * The new MPU regions are allocated per type after the statically
+	 * configured regions. The type is one-indexed rather than
+	 * zero-indexed, therefore we need to subtract by one to get the region
+	 * index.
 	 */
-	u32_t region_index = mpu_config.num_regions + type;
+	u32_t region_index = mpu_config.num_regions + type - 1;
 	u32_t region_attr = _get_region_attr_by_type(type, size);
 
 	/* ARM MPU supports up to 16 Regions */
diff --git a/arch/arm/core/cortex_m/mpu/nxp_mpu.c b/arch/arm/core/cortex_m/mpu/nxp_mpu.c
index 26312b1..54a638a 100644
--- a/arch/arm/core/cortex_m/mpu/nxp_mpu.c
+++ b/arch/arm/core/cortex_m/mpu/nxp_mpu.c
@@ -120,10 +120,12 @@
 {
 	SYS_LOG_DBG("Region info: 0x%x 0x%x", base, size);
 	/*
-	 * The new MPU regions are are allocated per type after the statically
-	 * configured regions.
+	 * The new MPU regions are allocated per type after the statically
+	 * configured regions. The type is one-indexed rather than
+	 * zero-indexed, therefore we need to subtract by one to get the region
+	 * index.
 	 */
-	u32_t region_index = mpu_config.num_regions + type;
+	u32_t region_index = mpu_config.num_regions + type - 1;
 	u32_t region_attr = _get_region_attr_by_type(type);
 	u32_t last_region = _get_num_regions() - 1;