diff --git a/collects/file/untgz.rkt b/collects/file/untgz.rkt
index 08e6461a5f..99329c4b06 100644
--- a/collects/file/untgz.rkt
+++ b/collects/file/untgz.rkt
@@ -25,20 +25,23 @@
        call-with-input-file*)
    in
    (lambda (in)
-     (define in2
+     (define-values (in2 wait)
        (cond
         [(and (= (peek-byte in 0) #o037) 
               (= (peek-byte in 1) #o213))
          (define-values (in2 out) (make-pipe 4096))
-         (thread
-          (lambda ()
-            (dynamic-wind
-                (lambda () (void))
-                (lambda () (gunzip-through-ports in out))
-                (lambda () (close-output-port out)))))
-         in2]
-        [else in]))
-     (untar in2 #:dest dest #:strip-count strip-count #:filter filter))))
+         (define t
+           (thread
+            (lambda ()
+              (dynamic-wind
+                  (lambda () (void))
+                  (lambda () (gunzip-through-ports in out))
+                  (lambda () (close-output-port out))))))
+         (values in2 (lambda () (thread-wait t)))]
+        [else (values in void)]))
+     (begin0
+      (untar in2 #:dest dest #:strip-count strip-count #:filter filter)
+      (wait)))))