diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 660512e49..22f8e7f10 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -491,7 +491,7 @@ void LocalDerivationGoal::startBuilder()
 
     /* Create a temporary directory where the build will take
        place. */
-    tmpDir = createTempDir(settings.buildDir.get().value_or(""), "nix-build-" + std::string(drvPath.name()), false, false, 0700);
+    tmpDir = createTempDir(settings.buildDir.get(), "nix-build-" + std::string(drvPath.name()), false, false, 0700);
 
     chownToBuilder(tmpDir);
 
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 10d43ee3e..209a4417b 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1494,7 +1494,7 @@ std::pair<Path, AutoCloseFD> LocalStore::createTempDirInStore()
         /* There is a slight possibility that `tmpDir' gets deleted by
            the GC between createTempDir() and when we acquire a lock on it.
            We'll repeat until 'tmpDir' exists and we've locked it. */
-        tmpDirFn = createTempDir(realStoreDir, "tmp");
+        tmpDirFn = createTempDir(std::optional<Path>(realStoreDir), "tmp");
         tmpDirFd = AutoCloseFD{open(tmpDirFn.c_str(), O_RDONLY | O_DIRECTORY)};
         if (tmpDirFd.get() < 0) {
             continue;
diff --git a/src/libstore/ssh.cc b/src/libstore/ssh.cc
index 0d7bfa01d..0d9a4b421 100644
--- a/src/libstore/ssh.cc
+++ b/src/libstore/ssh.cc
@@ -20,7 +20,7 @@ SSHMaster::SSHMaster(const std::string & host, const std::string & keyFile, cons
         throw Error("invalid SSH host name '%s'", host);
 
     auto state(state_.lock());
-    state->tmpDir = std::make_unique<AutoDelete>(createTempDir("", "nix", true, true, 0700));
+    state->tmpDir = std::make_unique<AutoDelete>(createTempDir(std::optional<Path>(), "nix", true, true, 0700));
 }
 
 void SSHMaster::addCommonSSHOpts(Strings & args)
diff --git a/src/libutil/file-system.cc b/src/libutil/file-system.cc
index f0199d36f..fe4bf2807 100644
--- a/src/libutil/file-system.cc
+++ b/src/libutil/file-system.cc
@@ -531,17 +531,17 @@ std::string defaultTempDir() {
     return getEnvNonEmpty("TMPDIR").value_or("/tmp");
 }
 
-static Path tempName(Path tmpRoot, const Path & prefix, bool includePid,
+static Path tempName(const std::optional<Path> & tmpRoot, const Path & prefix, bool includePid,
     std::atomic<unsigned int> & counter)
 {
-    tmpRoot = canonPath(tmpRoot.empty() ? defaultTempDir() : tmpRoot, true);
+    auto canonTmpRoot = canonPath(tmpRoot ? *tmpRoot : defaultTempDir(), true);
     if (includePid)
-        return fmt("%1%/%2%-%3%-%4%", tmpRoot, prefix, getpid(), counter++);
+        return fmt("%1%/%2%-%3%-%4%", canonTmpRoot, prefix, getpid(), counter++);
     else
-        return fmt("%1%/%2%-%3%", tmpRoot, prefix, counter++);
+        return fmt("%1%/%2%-%3%", canonTmpRoot, prefix, counter++);
 }
 
-Path createTempDir(const Path & tmpRoot, const Path & prefix,
+Path createTempDir(const std::optional<Path> & tmpRoot, const Path & prefix,
     bool includePid, bool useGlobalCounter, mode_t mode)
 {
     static std::atomic<unsigned int> globalCounter = 0;
@@ -680,7 +680,7 @@ void moveFile(const Path & oldName, const Path & newName)
         auto newPath = fs::path(newName);
         // For the move to be as atomic as possible, copy to a temporary
         // directory
-        fs::path temp = createTempDir(newPath.parent_path(), "rename-tmp");
+        fs::path temp = createTempDir(std::optional(newPath.parent_path()), "rename-tmp");
         Finally removeTemp = [&]() { fs::remove(temp); };
         auto tempCopyTarget = temp / "copy-target";
         if (e.code().value() == EXDEV) {
diff --git a/src/libutil/file-system.hh b/src/libutil/file-system.hh
index e49323e84..eecef0ef5 100644
--- a/src/libutil/file-system.hh
+++ b/src/libutil/file-system.hh
@@ -266,8 +266,9 @@ typedef std::unique_ptr<DIR, DIRDeleter> AutoCloseDir;
 /**
  * Create a temporary directory.
  */
-Path createTempDir(const Path & tmpRoot = "", const Path & prefix = "nix",
-    bool includePid = true, bool useGlobalCounter = true, mode_t mode = 0755);
+Path createTempDir(const std::optional<Path> & tmpRoot = std::optional<Path>(),
+    const Path & prefix = "nix", bool includePid = true, bool useGlobalCounter = true,
+    mode_t mode = 0755);
 
 /**
  * Create a temporary file, returning a file handle and its path.
diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index 4b8d7a2fa..3d35ee47f 100644
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -53,7 +53,7 @@ static void main_nix_build(int argc, char * * argv)
     std::string script;
     std::vector<std::string> savedArgs;
 
-    AutoDelete tmpDir(createTempDir("", myName));
+    AutoDelete tmpDir(createTempDir(std::optional<Path>(), myName));
 
     std::string outLink = "./result";
 
diff --git a/src/nix/develop.cc b/src/nix/develop.cc
index fb144c904..9169f44f8 100644
--- a/src/nix/develop.cc
+++ b/src/nix/develop.cc
@@ -557,7 +557,7 @@ struct CmdDevelop : Common, MixEnvironment
 
         auto [rcFileFd, rcFilePath] = createTempFile("nix-shell");
 
-        AutoDelete tmpDir(createTempDir("", "nix-develop"), true);
+        AutoDelete tmpDir(createTempDir(std::optional<Path>(), "nix-develop"), true);
 
         auto script = makeRcScript(store, buildEnvironment, (Path) tmpDir);
 
@@ -694,7 +694,7 @@ struct CmdPrintDevEnv : Common, MixJSON
         if (json) {
             logger->writeToStdout(buildEnvironment.toJSON());
         } else {
-            AutoDelete tmpDir(createTempDir("", "nix-dev-env"), true);
+            AutoDelete tmpDir(createTempDir(std::optional<Path>(), "nix-dev-env"), true);
             logger->writeToStdout(makeRcScript(store, buildEnvironment, tmpDir));
         }
     }