diff --git a/src/libexpr/paths.cc b/src/libexpr/paths.cc
index 7e409738a..87aee2af0 100644
--- a/src/libexpr/paths.cc
+++ b/src/libexpr/paths.cc
@@ -52,7 +52,7 @@ SourcePath EvalState::decodePath(std::string_view s, PosIdx pos)
             SourcePath path {accessor->second, CanonPath(s)};
 
             static bool warned = false;
-            warnOnce(warned, fmt("applying 'toString' to path '%s' and then accessing it is deprecated, at %s", path, positions[pos]));
+            warnOnce(warned, "applying 'toString' to path '%s' and then accessing it is deprecated, at %s", path, positions[pos]);
 
             return path;
         } catch (std::invalid_argument & e) {
diff --git a/src/libutil/logging.cc b/src/libutil/logging.cc
index a8ab78546..9702de1ad 100644
--- a/src/libutil/logging.cc
+++ b/src/libutil/logging.cc
@@ -105,14 +105,6 @@ public:
 
 Verbosity verbosity = lvlInfo;
 
-void warnOnce(bool & haveWarned, const FormatOrString & fs)
-{
-    if (!haveWarned) {
-        warn(fs.s);
-        haveWarned = true;
-    }
-}
-
 void writeToStderr(std::string_view s)
 {
     try {
diff --git a/src/libutil/logging.hh b/src/libutil/logging.hh
index ae7a5917b..4642c49f7 100644
--- a/src/libutil/logging.hh
+++ b/src/libutil/logging.hh
@@ -225,7 +225,11 @@ inline void warn(const std::string & fs, const Args & ... args)
     logger->warn(f.str());
 }
 
-void warnOnce(bool & haveWarned, const FormatOrString & fs);
+#define warnOnce(haveWarned, args...) \
+    if (!haveWarned) {                \
+        haveWarned = true;            \
+        warn(args);                   \
+    }
 
 void writeToStderr(std::string_view s);