diff --git a/include/boost/unordered/detail/implementation.hpp b/include/boost/unordered/detail/implementation.hpp
index a7a2d186..aa77aa39 100644
--- a/include/boost/unordered/detail/implementation.hpp
+++ b/include/boost/unordered/detail/implementation.hpp
@@ -492,7 +492,7 @@ template <typename A0> struct emplace_args1
 {
     BOOST_UNORDERED_EARGS_MEMBER(1, 0, _)
 
-    emplace_args1(Arg0 b0) : a0(b0) {}
+    explicit emplace_args1(Arg0 b0) : a0(b0) {}
 };
 
 template <typename A0>