diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 0349ce6e7..6a4afe8da 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -542,48 +542,59 @@ std::optional<Path> getSourceFilePathFromInStorePath(Store & store, const Path &
 
     return betterFilePath;
 }
+
+std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> toDerivationImpl(InstallableFlake & flake) {
+    auto lockedFlake = flake.getLockedFlake();
+
+    auto cache = openEvalCache(*flake.state, lockedFlake);
+    auto root = cache->getRoot();
+
+    for (auto & attrPath : flake.getActualAttrPaths()) {
+        auto attr = root->findAlongAttrPath(
+            parseAttrPath(*flake.state, attrPath),
+            true
+        );
+
+        if (!attr) continue;
+
+        if (!attr->isDerivation())
+            throw Error("flake output attribute '%s' is not a derivation", attrPath);
+
+        auto drvPath = attr->forceDerivation();
+
+        auto drvInfo = InstallableValue::DerivationInfo{
+            std::move(drvPath),
+            flake.state->store->maybeParseStorePath(attr->getAttr(flake.state->sOutPath)->getString()),
+            attr->getAttr(flake.state->sOutputName)->getString()
+        };
+
+        return {attrPath, lockedFlake->flake.lockedRef, std::move(drvInfo)};
+    }
+
+    throw Error("flake '%s' does not provide attribute %s",
+        flake.flakeRef, showAttrPaths(flake.getActualAttrPaths()));
 }
 
-std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableFlake::toDerivation()
-{
+template <class F>
+auto patchErrorSourceFile(Store & store, const fetchers::Input & flakeInput, F && func) {
     try {
-        auto lockedFlake = getLockedFlake();
-
-        auto cache = openEvalCache(*state, lockedFlake);
-        auto root = cache->getRoot();
-
-        for (auto & attrPath : getActualAttrPaths()) {
-            auto attr = root->findAlongAttrPath(
-                parseAttrPath(*state, attrPath),
-                true
-            );
-
-            if (!attr) continue;
-
-            if (!attr->isDerivation())
-                throw Error("flake output attribute '%s' is not a derivation", attrPath);
-
-            auto drvPath = attr->forceDerivation();
-
-            auto drvInfo = DerivationInfo{
-                std::move(drvPath),
-                state->store->maybeParseStorePath(attr->getAttr(state->sOutPath)->getString()),
-                attr->getAttr(state->sOutputName)->getString()
-            };
-
-            return {attrPath, lockedFlake->flake.lockedRef, std::move(drvInfo)};
-        }
+        return func();
     } catch (BaseError & e) {
         if (auto pos = e.info().errPos) {
-            if (auto sourceFilePath = getSourceFilePathFromInStorePath(*state->store, pos->file, flakeRef.input)) {
-                e.setBetterErrPosFile(*sourceFilePath);
+            if (auto sourceFilePath = getSourceFilePathFromInStorePath(store, pos->file, flakeInput)) {
+                e.replacePosFile(*sourceFilePath);
             }
         }
         throw;
     }
+}
+}
 
-    throw Error("flake '%s' does not provide attribute %s",
-        flakeRef, showAttrPaths(getActualAttrPaths()));
+std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableFlake::toDerivation()
+{
+    return patchErrorSourceFile(*state->store, flakeRef.input, [&] {
+        return toDerivationImpl(*this);
+    });
 }
 
 std::vector<InstallableValue::DerivationInfo> InstallableFlake::toDerivations()
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 4622fe4c6..6098d7732 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -165,7 +165,7 @@ public:
     const string & msg() const { return calcWhat(); }
     const ErrorInfo & info() const { calcWhat(); return err; }
     
-    void setBetterErrPosFile(std::string file) {
+    void replacePosFile(std::string file) {
         if (err.errPos) {
             err.errPos->file = std::move(file);
         }