diff --git a/core/net/mac/tsch/tsch-conf.h b/core/net/mac/tsch/tsch-conf.h
index bd9726042..3f877884a 100644
--- a/core/net/mac/tsch/tsch-conf.h
+++ b/core/net/mac/tsch/tsch-conf.h
@@ -135,7 +135,7 @@
 #define TSCH_DEFAULT_TS_MAX_TX             4256
 #define TSCH_DEFAULT_TS_TIMESLOT_LENGTH    15000
 
-#elif TSCH_CONF_DEFAULT_TIMESLOT_LENGTH == 65000
+#elif TSCH_CONF_DEFAULT_TIMESLOT_LENGTH == 65000U
 /* 65ms timeslot, i.e. nearly the max length allowed by standard (16-bit unsigned in micro-seconds).
  * Useful for running link-layer security on sky or z1 in Cooja, where only S/W security is supported.
  * Note: this slot timing would require a total of 120ms. If a slot overlaps with the next active slot,