diff --git a/vcfpp.h b/vcfpp.h
index 84823c0..354d657 100644
--- a/vcfpp.h
+++ b/vcfpp.h
@@ -1555,7 +1555,7 @@ class BcfReader
     std::shared_ptr<hts_itr_t> itr; // hts iterator
     kstring_t s = {0, 0, NULL}; // kstring
     std::string fname;
-    bool isBcf; // if the input file is bcf or vcf;
+    bool isBcf = false; // if the input file is bcf or vcf;
 
   public:
     /// a BcfHeader object