diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index d87d7b9b1..6db9bf9a1 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -443,10 +443,10 @@ ref<eval_cache::EvalCache> openEvalCache(
     EvalState & state,
     std::shared_ptr<flake::LockedFlake> lockedFlake)
 {
-    auto fingerprint = lockedFlake->getFingerprint();
+    auto fingerprint = lockedFlake->getFingerprint(state.store);
     return make_ref<nix::eval_cache::EvalCache>(
         evalSettings.useEvalCache && evalSettings.pureEval
-            ? std::optional { std::cref(fingerprint) }
+            ? fingerprint
             : std::nullopt,
         state,
         [&state, lockedFlake]()
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index fd9341504..4a69bb381 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -926,18 +926,17 @@ static RegisterPrimOp r4({
 
 }
 
-Fingerprint LockedFlake::getFingerprint() const
+std::optional<Fingerprint> LockedFlake::getFingerprint(ref<Store> store) const
 {
+    if (lockFile.isUnlocked()) return std::nullopt;
+
+    auto fingerprint = flake.lockedRef.input.getFingerprint(store);
+    if (!fingerprint) return std::nullopt;
+
     // FIXME: as an optimization, if the flake contains a lock file
     // and we haven't changed it, then it's sufficient to use
     // flake.sourceInfo.storePath for the fingerprint.
-    return hashString(HashAlgorithm::SHA256,
-        fmt("%s;%s;%d;%d;%s",
-            flake.path.to_string(),
-            flake.lockedRef.subdir,
-            flake.lockedRef.input.getRevCount().value_or(0),
-            flake.lockedRef.input.getLastModified().value_or(0),
-            lockFile));
+    return hashString(HashAlgorithm::SHA256, fmt("%s;%s;%s", *fingerprint, flake.lockedRef.subdir, lockFile));
 }
 
 Flake::~Flake() { }
diff --git a/src/libexpr/flake/flake.hh b/src/libexpr/flake/flake.hh
index 48907813f..1ba085f0f 100644
--- a/src/libexpr/flake/flake.hh
+++ b/src/libexpr/flake/flake.hh
@@ -119,7 +119,7 @@ struct LockedFlake
      */
     std::map<ref<Node>, SourcePath> nodePaths;
 
-    Fingerprint getFingerprint() const;
+    std::optional<Fingerprint> getFingerprint(ref<Store> store) const;
 };
 
 struct LockFlags