diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index 40354cc87..a5571d4ec 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -8,6 +8,11 @@ namespace nix
 
 std::optional<string> ErrorInfo::programName = std::nullopt;
 
+std::ostream& operator<<(std::ostream &os, const hintformat &hf)
+{
+    return os << hf.str();
+}
+
 string showErrPos(const ErrPos &errPos)
 {
     if (errPos.column > 0) {
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 8286eaab1..5658e6335 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -92,10 +92,7 @@ private:
     format fmt;
 };
 
-std::ostream& operator<<(std::ostream &os, const hintformat &hf)
-{
-    return os << hf.str();
-}
+std::ostream& operator<<(std::ostream &os, const hintformat &hf);
 
 template<typename... Args>
 inline hintformat hintfmt(const std::string & fs, const Args & ... args)