diff --git a/src/tawashi_implem/num_conv.hpp b/src/tawashi_implem/num_conv.hpp
new file mode 100644
index 0000000..a3f1357
--- /dev/null
+++ b/src/tawashi_implem/num_conv.hpp
@@ -0,0 +1,34 @@
+/* Copyright 2017, Michele Santullo
+ * This file is part of "tawashi".
+ *
+ * "tawashi" is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * "tawashi" is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with "tawashi".  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include <ciso646>
+#include <duckhandy/lexical_cast.hpp>
+
+namespace tawashi {
+	template <typename T>
+	inline bool seems_valid_number (const boost::string_ref& parValue) {
+		const std::size_t skip_sign = (sprout::is_signed<T>::value and not parValue.empty() and parValue[0] == '-' ? 1 : 0);
+		for (std::size_t z = skip_sign; z < parValue.size(); ++z) {
+			const char c = parValue[z];
+			if (c < '0' or c > '9')
+				return false;
+		}
+		return parValue.size() - skip_sign <= dhandy::tags::dec<T>::count_digits_bt(sprout::numeric_limits<T>::max());
+	}
+} //namespace tawashi
diff --git a/src/tawashi_implem/response.cpp b/src/tawashi_implem/response.cpp
index d3d65c6..f832ee4 100644
--- a/src/tawashi_implem/response.cpp
+++ b/src/tawashi_implem/response.cpp
@@ -24,6 +24,7 @@
 #include "pathname/pathname.hpp"
 #include "list_highlight_langs.hpp"
 #include "cgi_env.hpp"
+#include "num_conv.hpp"
 #include <utility>
 #include <cassert>
 #include <fstream>
@@ -126,7 +127,7 @@ namespace tawashi {
 						oss << ':' << port;
 					}
 				}
-				else if (not host_port.empty()) {
+				else if (not host_port.empty() and seems_valid_number<uint16_t>(host_port)) {
 					oss << ':' << host_port;
 				}
 			}