diff --git a/makefiles/boot/riotboot.mk b/makefiles/boot/riotboot.mk
index 8cfac6b0a025..447c9ac08be3 100644
--- a/makefiles/boot/riotboot.mk
+++ b/makefiles/boot/riotboot.mk
@@ -86,6 +86,7 @@ riotboot/flash-bootloader: riotboot/bootloader/flash
 riotboot/bootloader/%: $$(if $$(filter riotboot/bootloader/clean,$$@),,$$(BUILDDEPS) pkg-prepare)
 	$(Q)/usr/bin/env -i \
 		QUIET=$(QUIET) PATH="$(PATH)" USER="$(USER)"\
+		INCLUDES="$(INCLUDES)"\
 		EXTERNAL_BOARD_DIRS="$(EXTERNAL_BOARD_DIRS)" BOARD=$(BOARD)\
 		DEBUG_ADAPTER_ID=$(DEBUG_ADAPTER_ID) \
 		IOTLAB_NODE=$(IOTLAB_NODE) \
diff --git a/sys/include/riotboot/slot.h b/sys/include/riotboot/slot.h
index ba44c44c1c9d..f49da61c9900 100644
--- a/sys/include/riotboot/slot.h
+++ b/sys/include/riotboot/slot.h
@@ -116,8 +116,10 @@ void riotboot_slot_dump_addrs(void);
 static inline size_t riotboot_slot_size(unsigned slot)
 {
     switch (slot) {
+#if NUM_SLOTS >= 1
     case 0:
         return SLOT0_LEN;
+#endif
 #if NUM_SLOTS == 2
     case 1:
         return SLOT1_LEN;