Index: std/range.d
===================================================================
--- std/range.d	(revision 1824)
+++ std/range.d	(working copy)
@@ -2549,6 +2549,7 @@
     this(N current, N pastLast, S step)
     {
         enforce(step != 0 && current != pastLast);
+        enforce((current < pastLast) == (step > 0), "iota: incorrect startup parameters");
         this.current = current;
         this.step = step;
         if (step > 0)