diff --git a/src/main/java/org/apache/maven/plugins/clean/CleanMojo.java b/src/main/java/org/apache/maven/plugins/clean/CleanMojo.java
index 0db1b3f..89cad6b 100644
--- a/src/main/java/org/apache/maven/plugins/clean/CleanMojo.java
+++ b/src/main/java/org/apache/maven/plugins/clean/CleanMojo.java
@@ -283,11 +283,12 @@ public void execute() {
         if (fast && session != null) {
             Path tmpDir = fastDir;
             if (tmpDir == null) {
-                Path rootDir;
+                Path rootDir = null;
                 try {
                     rootDir = session.getRootDirectory();
                 } catch (IllegalStateException e) {
-                    rootDir = null;
+                    // Embedded usage where no root directory is available — fall through to fallback.
+                    logger.debug("Missing root directory, falling back to system temp directory.", e);
                 }
                 tmpDir = rootDir != null
                         ? rootDir.resolve(".mvn").resolve("target").resolve("clean")