diff --git a/plugin/src/main/java/org/jenkinsci/plugins/workflow/cps/CpsFlowExecution.java b/plugin/src/main/java/org/jenkinsci/plugins/workflow/cps/CpsFlowExecution.java
index b79a42401..0ff38a6c3 100644
--- a/plugin/src/main/java/org/jenkinsci/plugins/workflow/cps/CpsFlowExecution.java
+++ b/plugin/src/main/java/org/jenkinsci/plugins/workflow/cps/CpsFlowExecution.java
@@ -557,13 +557,19 @@ private Env createInitialEnv() {
 
     private CpsScript parseScript() throws IOException {
         // classloader hierarchy. See doc/classloader.md
-        trusted = new CpsGroovyShellFactory(this).forTrusted().build();
-        shell = new CpsGroovyShellFactory(this).withParent(trusted).build();
+        CpsScript s;
+        try {
+            trusted = new CpsGroovyShellFactory(this).forTrusted().build();
+            shell = new CpsGroovyShellFactory(this).withParent(trusted).build();
 
-        CpsScript s = (CpsScript) shell.reparse("WorkflowScript",script);
+            s = (CpsScript) shell.reparse("WorkflowScript",script);
 
-        for (Entry<String, String> e : loadedScripts.entrySet()) {
-            shell.reparse(e.getKey(), e.getValue());
+            for (Entry<String, String> e : loadedScripts.entrySet()) {
+                shell.reparse(e.getKey(), e.getValue());
+            }
+        } catch (RuntimeException | Error x) {
+            closeShells();
+            throw x;
         }
 
         s.execution = this;
@@ -1304,20 +1310,26 @@ synchronized void onProgramEnd(Outcome outcome) {
         this.persistedClean = Boolean.TRUE;
     }
 
-    void cleanUpHeap() {
-        LOGGER.log(Level.FINE, "cleanUpHeap on {0}", owner);
+    private void closeShells() {
         try {
             if (shell != null) {
+                LOGGER.fine(() -> "closing main class loader from " + owner);
                 shell.getClassLoader().close();
                 shell = null;
             }
             if (trusted != null) {
+                LOGGER.fine(() -> "closing trusted class loader from " + owner);
                 trusted.getClassLoader().close();
                 trusted = null;
             }
         } catch (IOException x) {
             LOGGER.log(Level.WARNING, "failed to close class loaders from " + owner, x);
         }
+    }
+
+    void cleanUpHeap() {
+        LOGGER.log(Level.FINE, "cleanUpHeap on {0}", owner);
+        closeShells();
         if (scriptClass != null) {
             try {
                 cleanUpLoader(scriptClass.getClassLoader(), new HashSet<>(), new HashSet<>());