diff --git a/src/libcmd/installable-derived-path.cc b/src/libcmd/installable-derived-path.cc
index 6ecf54b7c..35f6c5bfe 100644
--- a/src/libcmd/installable-derived-path.cc
+++ b/src/libcmd/installable-derived-path.cc
@@ -47,7 +47,7 @@ InstallableDerivedPath InstallableDerivedPath::parse(
                 };
                 warn(
                     "The interpretation of store paths arguments ending in `.drv` recently changed. If this command is now failing try again with '%s'",
-                    oldDerivedPath.to_string(*store));
+                    oldDerivedPath.to_string(*store, '^'));
             };
             return DerivedPath::Opaque {
                 .path = std::move(storePath),
diff --git a/src/libstore/derived-path.cc b/src/libstore/derived-path.cc
index e5f0f1b33..6baca70a3 100644
--- a/src/libstore/derived-path.cc
+++ b/src/libstore/derived-path.cc
@@ -59,18 +59,19 @@ std::string DerivedPath::Opaque::to_string(const Store & store) const
     return store.printStorePath(path);
 }
 
-std::string DerivedPath::Built::to_string(const Store & store) const
+std::string DerivedPath::Built::to_string(const Store & store, char separator) const
 {
     return store.printStorePath(drvPath)
-        + "!"
+        + separator
         + outputs.to_string();
 }
 
-std::string DerivedPath::to_string(const Store & store) const
+std::string DerivedPath::to_string(const Store & store, char separator) const
 {
-    return std::visit(
-        [&](const auto & req) { return req.to_string(store); },
-        this->raw());
+    return std::visit(overloaded {
+        [&](const DerivedPath::Built & req) { return req.to_string(store, separator); },
+        [&](const DerivedPath::Opaque & req) { return req.to_string(store); },
+    }, this->raw());
 }
 
 
diff --git a/src/libstore/derived-path.hh b/src/libstore/derived-path.hh
index 2155776b1..9fc64e2f2 100644
--- a/src/libstore/derived-path.hh
+++ b/src/libstore/derived-path.hh
@@ -48,7 +48,7 @@ struct DerivedPathBuilt {
     StorePath drvPath;
     OutputsSpec outputs;
 
-    std::string to_string(const Store & store) const;
+    std::string to_string(const Store & store, char separator = '!') const;
     static DerivedPathBuilt parse(const Store & store, std::string_view, std::string_view);
     nlohmann::json toJSON(ref<Store> store) const;
 
@@ -81,7 +81,7 @@ struct DerivedPath : _DerivedPathRaw {
         return static_cast<const Raw &>(*this);
     }
 
-    std::string to_string(const Store & store) const;
+    std::string to_string(const Store & store, char separator = '!') const;
     static DerivedPath parse(const Store & store, std::string_view);
 };