diff --git a/av-variance.c b/av-variance.c
index bc314aa..4c6cb50 100644
--- a/av-variance.c
+++ b/av-variance.c
@@ -27,6 +27,7 @@ double sample_variance(double set[], int set_n) {
   }
 
   // Divide by (n-1), as this is *sample* variance (Bessel's correction).
+  assert(set_n>1);
   ss /= (set_n-1);
 
   return ss;