diff --git a/include/fc/crypto/bigint.hpp b/include/fc/crypto/bigint.hpp
index debc869..b91631a 100644
--- a/include/fc/crypto/bigint.hpp
+++ b/include/fc/crypto/bigint.hpp
@@ -21,7 +21,7 @@ namespace fc {
       bigint& operator = ( const bigint& a );
       bigint& operator = ( bigint&& a );
 
-      operator bool()const;
+      explicit operator bool()const;
 
       bool    is_negative()const;
       int64_t to_int64()const;