diff --git a/lib/KorAP/Tokenizer/Units.pm b/lib/KorAP/Tokenizer/Units.pm
index 8130cb9..baa9ed2 100644
--- a/lib/KorAP/Tokenizer/Units.pm
+++ b/lib/KorAP/Tokenizer/Units.pm
@@ -14,6 +14,8 @@
 
   ($from, $to) = $self->_offset($from, $to);
 
+  return if !$to;
+  $from ||= 0;
   return unless $to > $from;
 
   my $span = KorAP::Tokenizer::Span->new;