diff --git a/dist/index.js b/dist/index.js
index 704b9da..dba1be0 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -92706,8 +92706,11 @@ function envPreInstall() {
   if (windows) {
     // puts normal Ruby temp folder on SSD
     core.exportVariable('TMPDIR', ENV['RUNNER_TEMP'])
-    // bash - sets home to match native windows, normally C:\Users\<user name>
-    core.exportVariable('HOME', ENV['HOMEDRIVE'] + ENV['HOMEPATH'])
+    // Use os.homedir() which calls the Windows API (SHGetFolderPath) directly.
+    // Env-based approaches (USERPROFILE, HOMEDRIVE+HOMEPATH) are unreliable on some
+    // runners (e.g. Blacksmith) where these vars are unset, causing NaN which
+    // JSON.stringify serializes to "null".
+    core.exportVariable('HOME', os.homedir())
     // bash - needed to maintain Path from Windows
     core.exportVariable('MSYS2_PATH_TYPE', 'inherit')
   }
diff --git a/index.js b/index.js
index 7fdcf5b..fc8bbe6 100644
--- a/index.js
+++ b/index.js
@@ -219,8 +219,11 @@ function envPreInstall() {
   if (windows) {
     // puts normal Ruby temp folder on SSD
     core.exportVariable('TMPDIR', ENV['RUNNER_TEMP'])
-    // bash - sets home to match native windows, normally C:\Users\<user name>
-    core.exportVariable('HOME', ENV['HOMEDRIVE'] + ENV['HOMEPATH'])
+    // Use os.homedir() which calls the Windows API (SHGetFolderPath) directly.
+    // Env-based approaches (USERPROFILE, HOMEDRIVE+HOMEPATH) are unreliable on some
+    // runners (e.g. Blacksmith) where these vars are unset, causing NaN which
+    // JSON.stringify serializes to "null".
+    core.exportVariable('HOME', os.homedir())
     // bash - needed to maintain Path from Windows
     core.exportVariable('MSYS2_PATH_TYPE', 'inherit')
   }