diff --git a/hisat_3n_table.cpp b/hisat_3n_table.cpp
index f639677..a537228 100644
--- a/hisat_3n_table.cpp
+++ b/hisat_3n_table.cpp
@@ -287,7 +287,8 @@ int hisat_3n_table()
         }
         // if the samChromosome is different than current positions' chromosome, finish all SAM line.
         // then load a new reference chromosome.
-        if (samChromosome != *positions->chromosome) {
+        auto old = positions->chromosome;
+        if (samChromosome != (old ? *old : "")) {
             positions->appendingFinished();
             positions->moveAllToOutput();
             positions->loadNewChromosome(std::move(samChromosome));