diff --git a/src/libutil/include/nix/util/strings.hh b/src/libutil/include/nix/util/strings.hh
index da6decc31..da8409e6a 100644
--- a/src/libutil/include/nix/util/strings.hh
+++ b/src/libutil/include/nix/util/strings.hh
@@ -166,4 +166,10 @@ public:
     }
 };
 
+/**
+ * Check that the string does not contain any NUL bytes and return c_str().
+ * @throws Error if str contains '\0' bytes.
+ */
+const char * requireCString(const std::string & str);
+
 } // namespace nix
diff --git a/src/libutil/strings.cc b/src/libutil/strings.cc
index c0c3d6602..91a0f73ec 100644
--- a/src/libutil/strings.cc
+++ b/src/libutil/strings.cc
@@ -5,6 +5,7 @@
 #include "nix/util/strings-inline.hh"
 #include "nix/util/os-string.hh"
 #include "nix/util/error.hh"
+#include "nix/util/util.hh"
 
 namespace nix {
 
@@ -152,4 +153,14 @@ std::string optionalBracket(std::string_view prefix, std::string_view content, s
     return result;
 }
 
+const char * requireCString(const std::string & s)
+{
+    if (std::memchr(s.data(), '\0', s.size())) [[unlikely]] {
+        using namespace std::string_view_literals;
+        auto str = replaceStrings(s, "\0"sv, "␀"sv);
+        throw Error("string '%s' with null (\\0) bytes used where it's not allowed", str);
+    }
+    return s.c_str();
+}
+
 } // namespace nix
diff --git a/src/libutil/url.cc b/src/libutil/url.cc
index 72042901c..0a8b64528 100644
--- a/src/libutil/url.cc
+++ b/src/libutil/url.cc
@@ -327,8 +327,11 @@ Path renderUrlPathEnsureLegal(const std::vector<std::string> & urlPath)
         /* This is only really valid for UNIX. Windows has more restrictions. */
         if (comp.contains('/'))
             throw BadURL("URL path component '%s' contains '/', which is not allowed in file names", comp);
-        if (comp.contains(char(0)))
-            throw BadURL("URL path component '%s' contains NUL byte which is not allowed", comp);
+        if (comp.contains(char(0))) {
+            using namespace std::string_view_literals;
+            auto str = replaceStrings(comp, "\0"sv, "␀"sv);
+            throw BadURL("URL path component '%s' contains NUL byte which is not allowed", str);
+        }
     }
 
     return concatStringsSep("/", urlPath);