diff --git a/util.c b/util.c
index 2f4df15..6803c02 100644
--- a/util.c
+++ b/util.c
@@ -641,6 +641,7 @@ UTI_GetNtp64Fuzz(NTP_int64 *ts, int precision)
   int start, bits;
 
   assert(precision >= -32 && precision <= 32);
+  assert(sizeof (*ts) == 8);
 
   start = sizeof (*ts) - (precision + 32 + 7) / 8;
   ts->hi = ts->lo = 0;