diff --git a/modules/seq/base/src/impl/sequence_impl.cc b/modules/seq/base/src/impl/sequence_impl.cc
index d57989d9544441cf4c2d20e77dcb879d22dc2419..070211924e0c121bdd35abb61cc368090d8a7762 100644
--- a/modules/seq/base/src/impl/sequence_impl.cc
+++ b/modules/seq/base/src/impl/sequence_impl.cc
@@ -170,6 +170,8 @@ int SequenceImpl::GetLength() const {
 
 char SequenceImpl::GetOneLetterCode(int position) const
 {
+  if (position<0 || position>=static_cast<int>(seq_string_.length()))
+    throw Error("Position is not covered in sequence");
   return seq_string_[position];
 }