diff --git a/vcfpp.h b/vcfpp.h
index 19231ce..fd9acf5 100644
--- a/vcfpp.h
+++ b/vcfpp.h
@@ -1655,13 +1655,13 @@ class BcfReader
         {
             isBcf = false;
             tidx = tbx_index_load(fname.c_str());
-            if(tidx != NULL) throw std::runtime_error("error in loading tabix index!");
+            if(tidx == NULL) throw std::runtime_error("error in loading tabix index!");
             if(itr) tbx_itr_destroy(itr); // reset current region.
             if(region.empty())
                 itr = tbx_itr_querys(tidx, ".");
             else
                 itr = tbx_itr_querys(tidx, region.c_str());
-            if(itr != NULL) throw std::runtime_error("no interval region found!");
+            if(itr == NULL) throw std::runtime_error("no interval region found!");
         }
     }