fix change detection bug

diff --git a/lib/Text/Tradition/Collation.pm b/lib/Text/Tradition/Collation.pm
index 9ec858e..69b1a59 100644 (file)
--- a/lib/Text/Tradition/Collation.pm
+++ b/lib/Text/Tradition/Collation.pm
@@ -1366,7 +1366,8 @@ sub calculate_ranks {
     # Do we need to invalidate the cached data?
     if( $self->has_cached_svg || $self->has_cached_table ) {
        foreach my $r ( $self->readings ) {
-               next if $existing_ranks{$r} && $existing_ranks{$r} == $r->rank;
+               next if defined( $existing_ranks{$r} ) 
+                       && $existing_ranks{$r} == $r->rank;
                # Something has changed, so clear the cache
                $self->_clear_cache;
                        # ...and recalculate the common readings.