diff --git a/src/error-demo/error-demo.cc b/src/error-demo/error-demo.cc
index 82b03d71b..b1b313e32 100644
--- a/src/error-demo/error-demo.cc
+++ b/src/error-demo/error-demo.cc
@@ -35,6 +35,14 @@ int main()
         logger->logEI(ei);
     }
 
+    // SysError; picks up errno
+    try {
+        auto x = readFile(-1);
+    }
+    catch (Error &e) {
+        std::cout << "error: " << e.sname() << std::endl;
+        logError(e.info());
+    }
 
     // For completeness sake, info through vomit levels.
     // But this is maybe a heavy format for those.
diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index 91fa1ccd8..d4305ddd8 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -16,14 +16,6 @@ BaseError & BaseError::addPrefix(const FormatOrString & fs)
     return *this;
 }
 
-
-std::string SysError::addErrno(const std::string & s)
-{
-    errNo = errno;
-    return s + ": " + strerror(errNo);
-}
-
-
 std::optional<string> ErrorInfo::programName = std::nullopt;
 
 std::ostream& operator<<(std::ostream &os, const hintformat &hf)
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 86cff5609..2155ad344 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -160,13 +160,14 @@ public:
 
     template<typename... Args>
     SysError(const Args & ... args)
-        : Error(args...)  // TODO addErrNo for hintfmt
-          // : Error(addErrno(hintfmt(args...)))
-    { }
+      :Error("")
+    {
+        errNo = errno;
+        auto hf = hintfmt(args...);
+        err.hint = hintfmt("%1% : %2%", normaltxt(hf.str()), strerror(errNo));
+    }
 
-private:
-
-    std::string addErrno(const std::string & s);
+    virtual const char* sname() const override { return "SysError"; }
 };
 
 }
diff --git a/src/libutil/fmt.hh b/src/libutil/fmt.hh
index bfacfeb4d..12ab9c407 100644
--- a/src/libutil/fmt.hh
+++ b/src/libutil/fmt.hh
@@ -75,8 +75,8 @@ inline std::string fmt(const std::string & fs, const Args & ... args)
 template <class T>
 struct yellowtxt
 {
-    yellowtxt(T &s) : value(s) {}
-    T &value;
+    yellowtxt(const T &s) : value(s) {}
+    const T &value;
 };
 
 template <class T>
@@ -88,8 +88,8 @@ std::ostream& operator<<(std::ostream &out, const yellowtxt<T> &y)
 template <class T>
 struct normaltxt
 {
-    normaltxt(T &s) : value(s) {}
-    T &value;
+    normaltxt(const T &s) : value(s) {}
+    const T &value;
 };
 
 template <class T>
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 68dc1b738..e8f22ab71 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -312,7 +312,8 @@ string readFile(const Path & path, bool drain)
 void readFile(const Path & path, Sink & sink)
 {
     AutoCloseFD fd = open(path.c_str(), O_RDONLY | O_CLOEXEC);
-    if (!fd) throw SysError("opening file '%s'", path);
+    if (!fd)  
+        throw SysError("opening file '%s'", path);
     drainFD(fd.get(), sink);
 }