diff --git a/include/libtorrent/string_util.hpp b/include/libtorrent/string_util.hpp
index 916cc159e..a191b87fb 100644
--- a/include/libtorrent/string_util.hpp
+++ b/include/libtorrent/string_util.hpp
@@ -38,7 +38,8 @@ POSSIBILITY OF SUCH DAMAGE.
 namespace libtorrent
 {
 	TORRENT_EXTRA_EXPORT bool is_alpha(char c);
-	TORRENT_EXTRA_EXPORT bool is_digit(char c);
+	// this is used by bdecode_recursive's header file
+	TORRENT_EXPORT bool is_digit(char c);
 	TORRENT_EXTRA_EXPORT bool is_print(char c);
 	TORRENT_EXTRA_EXPORT bool is_space(char c);
 	TORRENT_EXTRA_EXPORT char to_lower(char c);