diff --git a/src/duphold.nim b/src/duphold.nim
index a7e0e53..a6e89c9 100644
--- a/src/duphold.nim
+++ b/src/duphold.nim
@@ -522,7 +522,7 @@ proc annotate*(snps:snpset, variant:Variant, sample_i:int) =
   var ci = innerCI(variant)
   var i = lowerBound(snps.starts, ci.left.int32, system.cmp)
   var n = 0
-  while snps.starts[i] < ci.right:
+  while i < snps.starts.len and snps.starts[i] < ci.right:
     dhgt[5 * sample_i + snps.nalts[i]] += 1
     n += 1
     i += 1