diff --git a/nselib/formulas.lua b/nselib/formulas.lua
index f2acc620e..25f3c9c54 100644
--- a/nselib/formulas.lua
+++ b/nselib/formulas.lua
@@ -136,7 +136,9 @@ function mean_stddev(t)
   local i, m, s, sigma
 
   if #t == 0 then
-    return 0, nil
+    return nil, nil
+  elseif #t == 1 then
+    return t[1], 0
   end
 
   m = t[1]