diff --git a/platform/sky/platform-conf.h b/platform/sky/platform-conf.h
index cb6ea0cba..5ba273047 100644
--- a/platform/sky/platform-conf.h
+++ b/platform/sky/platform-conf.h
@@ -80,7 +80,7 @@
 /* Types for clocks and uip_stats */
 typedef unsigned short uip_stats_t;
 typedef unsigned long clock_time_t;
-typedef unsigned long off_t;
+typedef long off_t;
 
 /* the low-level radio driver */
 #define NETSTACK_CONF_RADIO   cc2420_driver