diff --git a/examples/cuda-fanout/gpu-worker.lsp b/examples/cuda-fanout/gpu-worker.lsp
index 8eab693..07b27c8 100644
--- a/examples/cuda-fanout/gpu-worker.lsp
+++ b/examples/cuda-fanout/gpu-worker.lsp
@@ -215,22 +215,34 @@
 ;; Pull a numeric value out of a parsed portal expression for telemetry.
 ;; Walks (a b c ...) looking for (name <number>); returns #f if missing.
 (define (portal-find-number form name)
-  (let loop ((rest (cdr form)))
-    (cond
-      ((null? rest) #f)
-      ((and (pair? (car rest)) (eq? (car (car rest)) name)
-            (pair? (cdr (car rest))) (number? (car (cdr (car rest)))))
-       (car (cdr (car rest))))
-      (else (loop (cdr rest))))))
+  ;; Guard: form may arrive as #f when read-from-string sees an empty
+  ;; or malformed portal file (demo_ops crashed mid-write, OOM, etc).
+  ;; A bare (cdr #f) here historically crashed the gpu-worker with
+  ;; "error: not a pair: #f", taking the port-8320 listener down.
+  (cond
+    ((not (pair? form)) #f)
+    (else
+     (let loop ((rest (cdr form)))
+       (cond
+         ((null? rest) #f)
+         ((not (pair? rest)) #f)
+         ((and (pair? (car rest)) (eq? (car (car rest)) name)
+               (pair? (cdr (car rest))) (number? (car (cdr (car rest)))))
+          (car (cdr (car rest))))
+         (else (loop (cdr rest))))))))
 
 ;; Walk through (timing-ms (cpu-total X) (gpu-kernel Y)) shape.
 (define (portal-timing form which)
-  (let loop ((rest (cdr form)))
-    (cond
-      ((null? rest) #f)
-      ((and (pair? (car rest)) (eq? (car (car rest)) 'timing-ms))
-       (portal-find-number (car rest) which))
-      (else (loop (cdr rest))))))
+  (cond
+    ((not (pair? form)) #f)
+    (else
+     (let loop ((rest (cdr form)))
+       (cond
+         ((null? rest) #f)
+         ((not (pair? rest)) #f)
+         ((and (pair? (car rest)) (eq? (car (car rest)) 'timing-ms))
+          (portal-find-number (car rest) which))
+         (else (loop (cdr rest))))))))
 
 (define (handle-cuda-sim-ops-bin args)
   (let* ((ops-path (car args))