diff --git a/src/test/basics/StringUtilities_test.cpp b/src/test/basics/StringUtilities_test.cpp
index e12bc538577..4dc844c27a3 100644
--- a/src/test/basics/StringUtilities_test.cpp
+++ b/src/test/basics/StringUtilities_test.cpp
@@ -44,6 +44,12 @@ class StringUtilities_test : public beast::unit_test::Suite
         testUnHexFailure("123X");
         testUnHexFailure("V");
         testUnHexFailure("XRP");
+
+        testUnHexFailure(std::string(2, static_cast<char>(0xFF)));
+        testUnHexFailure(std::string(2, static_cast<char>(0x80)));
+        testUnHexFailure(std::string(1, static_cast<char>(0xFE)));
+        testUnHexFailure(std::string(1, static_cast<char>(0x80)));
+        testUnHexFailure(std::string("4") + static_cast<char>(0x80));
     }
 
     void