diff --git a/scheduler.scm b/scheduler.scm
index e31f6c8..7a0b4c6 100644
--- a/scheduler.scm
+++ b/scheduler.scm
@@ -384,17 +384,34 @@ EOF
 (define ##sys#fd-list '())		; ((FD1 THREAD1 ...) ...)

 (define (create-fdset)
+  (define (remove-empty ls)
+    (if (null? ls)
+      '()
+      (if (null? (cdar ls))
+        (remove-empty (cdr ls))
+        (cons (car ls) (remove-empty (cdr ls))))))
   ((foreign-lambda void "C_prepare_fdset" int) (##sys#length ##sys#fd-list))
-  (let loop ((lst ##sys#fd-list))
-    (unless (null? lst)
-      (let ((fd (caar lst)))
-	(for-each
-	 (lambda (t)
-	   (let ((p (##sys#slot t 11)))
-             (when (pair? p) ; (FD . RWFLAGS)? (can also be mutex or thread)
-               (fdset-set fd (cdr p)))))
-	 (cdar lst))
-	(loop (cdr lst))))))
+  ; Loop through ##sys#fd-list, calling fdset-set on the threaads blocking on
+  ; each fd. If a thread is not blocking on a fd (i.e. its block object in
+  ; slot 11 is not a (fd . rwflags) pair) then remove it from the list.
+  (for-each
+    (lambda (item)
+      (let* ((fd (car item)))
+        (set-cdr!
+          item
+          (let loop-ts ((ts (cdr item))
+                        (acc '()))
+            (if (null? ts)
+              acc
+              (let* ((t (car ts))
+                     (p (##sys#slot t 11)))
+                (if (pair? p) ; (FD . RWFLAGS)? (can also be mutex or thread)
+                  (begin
+                    (fdset-set fd (cdr p))
+                    (loop-ts (cdr ts) (cons t acc)))
+                  (loop-ts (cdr ts) acc))))))))
+    ##sys#fd-list)
+  (set! ##sys#fd-list (remove-empty ##sys#fd-list)))

 (define (fdset-set fd i/o)
   (let ((fdset-add! (foreign-lambda void "C_fdset_add" int bool bool)))