diff --git a/include/image-commands.mk b/include/image-commands.mk
index 01040138a4..f7e6ba6aa6 100644
--- a/include/image-commands.mk
+++ b/include/image-commands.mk
@@ -309,7 +309,7 @@ metadata_json = \
 
 define Build/append-metadata
 	$(if $(SUPPORTED_DEVICES),-echo $(call metadata_json,$(SUPPORTED_DEVICES)) | fwtool -I - $@)
-	[ -s "$(BUILD_KEY)" -a -s "$(BUILD_KEY).ucert" ] && { \
+	[ ! -s "$(BUILD_KEY)" -o ! -s "$(BUILD_KEY).ucert" ] || { \
 		cp "$(BUILD_KEY).ucert" "$@.ucert" ;\
 		usign -S -m "$@" -s "$(BUILD_KEY)" -x "$@.sig" ;\
 		ucert -A -c "$@.ucert" -x "$@.sig" ;\