diff --git a/include/libtorrent/span.hpp b/include/libtorrent/span.hpp
index 6d635eb37..abde53310 100644
--- a/include/libtorrent/span.hpp
+++ b/include/libtorrent/span.hpp
@@ -78,6 +78,11 @@ namespace aux {
 		span(std::array<U, N>& arr) noexcept // NOLINT
 			: m_ptr(arr.data()), m_len(arr.size()) {}
 
+		// this is necessary until C++17, where data() returns a non-const pointer
+		template <typename U>
+		span(std::basic_string<U>& str) noexcept // NOLINT
+			: m_ptr(&str[0]), m_len(str.size()) {}
+
 		template <typename U, std::size_t N>
 		span(U (&arr)[N]) noexcept // NOLINT
 			: m_ptr(&arr[0]), m_len(N) {}
diff --git a/include/libtorrent/string_util.hpp b/include/libtorrent/string_util.hpp
index ab6effbdf..80d6a00b7 100644
--- a/include/libtorrent/string_util.hpp
+++ b/include/libtorrent/string_util.hpp
@@ -68,7 +68,7 @@ namespace libtorrent {
 	TORRENT_EXTRA_EXPORT bool string_begins_no_case(char const* s1, char const* s2);
 	TORRENT_EXTRA_EXPORT bool string_equal_no_case(string_view s1, string_view s2);
 
-	TORRENT_EXTRA_EXPORT void url_random(char* begin, char* end);
+	TORRENT_EXTRA_EXPORT void url_random(span<char> dest);
 
 	TORRENT_EXTRA_EXPORT bool string_ends_with(string_view s1, string_view s2);
 
diff --git a/src/session_impl.cpp b/src/session_impl.cpp
index de0b2a157..e1794d0bf 100644
--- a/src/session_impl.cpp
+++ b/src/session_impl.cpp
@@ -1072,7 +1072,7 @@ namespace {
 			ret.upload_limit = 0xf0f0f0f;
 			ret.download_limit = 0xf0f0f0f;
 			ret.label.resize(20);
-			url_random(&ret.label[0], &ret.label[0] + 20);
+			url_random(span<char>(ret.label));
 			ret.ignore_unchoke_slots = false;
 			ret.connection_limit_factor = 0xf0f0f0f;
 			ret.upload_priority = 0xf0f0f0f;
@@ -5349,13 +5349,13 @@ namespace {
 	{
 		// ---- generate a peer id ----
 		std::string print = m_settings.get_str(settings_pack::peer_fingerprint);
-		if (print.size() > 20) print.resize(20);
+		if (print.size() > m_peer_id.size()) print.resize(m_peer_id.size());
 
 		// the client's fingerprint
 		std::copy(print.begin(), print.begin() + int(print.length()), m_peer_id.begin());
-		if (print.length() < 20)
+		if (print.size() < m_peer_id.size())
 		{
-			url_random(m_peer_id.data() + print.length(), m_peer_id.data() + 20);
+			url_random(span<char>(m_peer_id).subspan(print.length()));
 		}
 	}
 
@@ -6395,7 +6395,7 @@ namespace {
 		}
 
 		if (m_upnp) m_upnp->set_user_agent("");
-		url_random(m_peer_id.data(), m_peer_id.data() + 20);
+		url_random(m_peer_id);
 	}
 
 	void session_impl::update_force_proxy()
diff --git a/src/string_util.cpp b/src/string_util.cpp
index 72ceffdb3..26b36fe9b 100644
--- a/src/string_util.cpp
+++ b/src/string_util.cpp
@@ -129,7 +129,7 @@ namespace libtorrent {
 	}
 
 	// generate a url-safe random string
-	void url_random(char* begin, char* end)
+	void url_random(span<char> dest)
 	{
 		// http-accepted characters:
 		// excluding ', since some buggy trackers don't support that
@@ -137,8 +137,8 @@ namespace libtorrent {
 			"abcdefghijklmnopqrstuvwxyz-_.!~*()";
 
 		// the random number
-		while (begin != end)
-			*begin++ = printable[random(sizeof(printable) - 2)];
+		for (char& c : dest)
+			c = printable[random(sizeof(printable) - 2)];
 	}
 
 	bool string_ends_with(string_view s1, string_view s2)