diff --git a/sprout/checksum/md5.hpp b/sprout/checksum/md5.hpp
index 54519f60..8e76bdd9 100644
--- a/sprout/checksum/md5.hpp
+++ b/sprout/checksum/md5.hpp
@@ -22,6 +22,7 @@
 #include <sprout/iterator/bytes_iterator.hpp>
 #include <sprout/operation/fixed/set.hpp>
 #include <sprout/bit/rotl.hpp>
+#include <sprout/detail/udl_namespace.hpp>
 
 namespace sprout {
 	static_assert(CHAR_BIT == 8, "CHAR_BIT == 8");
diff --git a/sprout/checksum/sha1.hpp b/sprout/checksum/sha1.hpp
index 76fa81b6..d282b39f 100644
--- a/sprout/checksum/sha1.hpp
+++ b/sprout/checksum/sha1.hpp
@@ -26,6 +26,7 @@
 #include <sprout/range/algorithm/fixed/copy.hpp>
 #include <sprout/operation/fixed/set.hpp>
 #include <sprout/bit/rotl.hpp>
+#include <sprout/detail/udl_namespace.hpp>
 #ifdef SPROUT_WORKAROUND_NOT_TERMINATE_RECURSIVE_CONSTEXPR_FUNCTION_TEMPLATE
 #	include <sprout/workaround/recursive_function_template.hpp>
 #endif
diff --git a/sprout/checksum/sum.hpp b/sprout/checksum/sum.hpp
index 2268ff68..dc57be19 100644
--- a/sprout/checksum/sum.hpp
+++ b/sprout/checksum/sum.hpp
@@ -21,6 +21,7 @@
 #include <sprout/iterator/bytes_iterator.hpp>
 #include <sprout/numeric/accumulate.hpp>
 #include <sprout/detail/integer.hpp>
+#include <sprout/detail/udl_namespace.hpp>
 
 namespace sprout {
 	static_assert(CHAR_BIT == 8, "CHAR_BIT == 8");
diff --git a/sprout/checksum/xor.hpp b/sprout/checksum/xor.hpp
index 07d1f22e..9c56d6e0 100644
--- a/sprout/checksum/xor.hpp
+++ b/sprout/checksum/xor.hpp
@@ -18,6 +18,7 @@
 #include <sprout/iterator/bytes_iterator.hpp>
 #include <sprout/numeric/accumulate.hpp>
 #include <sprout/functional/polymorphic/bit_xor.hpp>
+#include <sprout/detail/udl_namespace.hpp>
 
 namespace sprout {
 	//