diff --git a/include/mcl/gmp_util.hpp b/include/mcl/gmp_util.hpp
index 3da87e4..384c052 100644
--- a/include/mcl/gmp_util.hpp
+++ b/include/mcl/gmp_util.hpp
@@ -651,6 +651,7 @@ void getNAFwidth(bool *pb, Vec& naf, mpz_class x, size_t w)
 			naf[i] = -naf[i];
 		}
 	}
+	*pb = true;
 }
 
 #ifndef CYBOZU_DONT_USE_EXCEPTION