diff --git a/src/libflake/flake/flake.cc b/src/libflake/flake/flake.cc
index 19b622a34af..2165ffd8dc9 100644
--- a/src/libflake/flake/flake.cc
+++ b/src/libflake/flake/flake.cc
@@ -672,6 +672,8 @@ LockedFlake lockFlake(
             if (lockFlags.writeLockFile) {
                 if (sourcePath || lockFlags.outputLockFilePath) {
                     if (auto unlockedInput = newLockFile.isUnlocked()) {
+                        if (lockFlags.failOnUnlocked)
+                            throw Error("cannot write lock file of flake '%s' because it has an unlocked input ('%s').\n", topRef, *unlockedInput);
                         if (state.fetchSettings.warnDirty)
                             warn("will not write lock file of flake '%s' because it has an unlocked input ('%s')", topRef, *unlockedInput);
                     } else {
diff --git a/src/libflake/flake/flake.hh b/src/libflake/flake/flake.hh
index cc2bea76e59..0dfd9440d6e 100644
--- a/src/libflake/flake/flake.hh
+++ b/src/libflake/flake/flake.hh
@@ -156,6 +156,11 @@ struct LockFlags
      */
     bool writeLockFile = true;
 
+    /**
+     * Throw an exception when the flake has an unlocked input.
+     */
+    bool failOnUnlocked = false;
+
     /**
      * Whether to use the registries to lookup indirect flake
      * references like 'nixpkgs'.
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 9f3584a11ac..55aa8971e12 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -162,6 +162,7 @@ struct CmdFlakeLock : FlakeCommand
         settings.tarballTtl = 0;
 
         lockFlags.writeLockFile = true;
+        lockFlags.failOnUnlocked = true;
         lockFlags.applyNixConfig = true;
 
         lockFlake();