diff --git a/libsel4debug/src/caps.c b/libsel4debug/src/caps.c
index c29b304..84b2731 100644
--- a/libsel4debug/src/caps.c
+++ b/libsel4debug/src/caps.c
@@ -14,7 +14,7 @@
 {
 #ifdef CONFIG_DEBUG_BUILD
     int type = seL4_DebugCapIdentify(cap);
-    printf("Cap %d has type %d\n", cap, type);
+    printf("Cap %"SEL4_PRIu_word" has type %d\n", cap, type);
 #else
     printf("DEBUG_BUILD not set, can't get type of cap %d", cap);
 #endif