diff --git a/sys/include/riotboot/slot.h b/sys/include/riotboot/slot.h
index ba44c44c1c..f49da61c99 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;