diff --git a/module/srfi/srfi-18.scm b/module/srfi/srfi-18.scm
index 79aedb8d1..7da6254d4 100644
--- a/module/srfi/srfi-18.scm
+++ b/module/srfi/srfi-18.scm
@@ -307,17 +307,19 @@ object (absolute point in time), or #f."
                         (timeout %unlock-sentinel))
   (let ((timeout (timeout->absolute-time timeout)))
     (when (mutex-owner mutex)
-      (set-mutex-owner! mutex #f)
       (cond
        ((eq? cond-var %unlock-sentinel)
+        (set-mutex-owner! mutex #f)
         (threads:unlock-mutex (mutex-prim mutex)))
        ((eq? timeout %unlock-sentinel)
         (threads:wait-condition-variable (condition-variable-prim cond-var)
                                          (mutex-prim mutex))
+        (set-mutex-owner! mutex #f)
         (threads:unlock-mutex (mutex-prim mutex)))
        ((threads:wait-condition-variable (condition-variable-prim cond-var)
                                          (mutex-prim mutex)
                                          timeout)
+        (set-mutex-owner! mutex #f)
         (threads:unlock-mutex (mutex-prim mutex)))
        (else #f)))))