diff --git a/boot/zephyr/single_loader.c b/boot/zephyr/single_loader.c
index 5d1e76f..75374d2 100644
--- a/boot/zephyr/single_loader.c
+++ b/boot/zephyr/single_loader.c
@@ -58,6 +58,7 @@
 }
 #endif /* MCUBOOT_VALIDATE_PRIMARY_SLOT || MCUBOOT_VALIDATE_PRIMARY_SLOT_ONCE*/
 
+#if defined(MCUBOOT_VALIDATE_PRIMARY_SLOT_ONCE)
 inline static fih_ret
 boot_image_validate_once(const struct flash_area *fa_p,
                     struct image_header *hdr)
@@ -88,6 +89,7 @@
     }
     FIH_RET(FIH_SUCCESS);
 }
+#endif
 
 /**
  * Gather information on image and prepare for booting.