diff --git a/include/nuttx/semaphore.h b/include/nuttx/semaphore.h
index 769da8baa4b..4178a51b474 100644
--- a/include/nuttx/semaphore.h
+++ b/include/nuttx/semaphore.h
@@ -74,6 +74,22 @@
 
 #define NXSEM_MHOLDER(s)      ((FAR atomic_t *)&((s)->val.mholder))
 
+/* Make a mutex holder value from a task id.  The holder field keeps it in
+ * the low 31 bits because bit 31 is the blocking flag, and a task id can
+ * be negative here: nxsched_gettid() reports -ESRCH for a context that no
+ * longer maps to a task, which is exactly the state a task is in while
+ * nxtask_exit() tears it down and its group's mutexes get their final
+ * lock and unlock.  Storing such an id unmasked would raise the blocking
+ * flag by accident, and an unlock would then try to wake waiters that do
+ * not exist.  Masking keeps a lock and unlock from the same context
+ * consistent with each other, whatever the id's sign.
+ *
+ * The ids -1 and -2 would alias NXSEM_MRESET and NXSEM_NO_MHOLDER, but
+ * nxsched_gettid() produces only valid ids and -ESRCH, which is -3.
+ */
+
+#define NXSEM_MAKE_MHOLDER(tid) ((uint32_t)(tid) & ~NXSEM_MBLOCKING_BIT)
+
 /* Check if holder value (TID) is not NO_HOLDER or RESET */
 
 #define NXSEM_MACQUIRED(h)    (((h) & NXSEM_NO_MHOLDER) != NXSEM_NO_MHOLDER)
diff --git a/libs/libc/semaphore/sem_trywait.c b/libs/libc/semaphore/sem_trywait.c
index 8a62ac9bd88..a7a8bd39894 100644
--- a/libs/libc/semaphore/sem_trywait.c
+++ b/libs/libc/semaphore/sem_trywait.c
@@ -152,7 +152,7 @@ int nxsem_trywait(FAR sem_t *sem)
               return -EAGAIN;
             }
 
-          new = _SCHED_GETTID();
+          new = NXSEM_MAKE_MHOLDER(_SCHED_GETTID());
         }
       else
         {
diff --git a/libs/libc/semaphore/sem_wait.c b/libs/libc/semaphore/sem_wait.c
index 36fb106bfc5..20b73a02f95 100644
--- a/libs/libc/semaphore/sem_wait.c
+++ b/libs/libc/semaphore/sem_wait.c
@@ -180,7 +180,7 @@ int nxsem_wait(FAR sem_t *sem)
               break;
             }
 
-          new = _SCHED_GETTID();
+          new = NXSEM_MAKE_MHOLDER(_SCHED_GETTID());
         }
       else
         {
diff --git a/sched/semaphore/sem_post.c b/sched/semaphore/sem_post.c
index 3ffa4e8e890..8018a1d869a 100644
--- a/sched/semaphore/sem_post.c
+++ b/sched/semaphore/sem_post.c
@@ -99,11 +99,14 @@ int nxsem_post_slow(FAR sem_t *sem)
       mholder = atomic_fetch_or(NXSEM_MHOLDER(sem), NXSEM_MBLOCKING_BIT);
 
       /* Mutex post from another thread is not allowed, unless
-       * called from nxsem_reset
+       * called from nxsem_reset.  The comparison uses the same encoding
+       * as the lock side so that a context whose id is -ESRCH, which is
+       * what a task being torn down reports, still matches its own lock.
        */
 
       DEBUGASSERT(mholder == (NXSEM_MBLOCKING_BIT | NXSEM_MRESET) ||
-                  (mholder & (~NXSEM_MBLOCKING_BIT)) == nxsched_gettid());
+                  (mholder & (~NXSEM_MBLOCKING_BIT)) ==
+                  NXSEM_MAKE_MHOLDER(nxsched_gettid()));
 
       blocking = NXSEM_MBLOCKING(mholder);
 
diff --git a/sched/semaphore/sem_trywait.c b/sched/semaphore/sem_trywait.c
index d8ec4b3d96c..40f2e10275c 100644
--- a/sched/semaphore/sem_trywait.c
+++ b/sched/semaphore/sem_trywait.c
@@ -88,7 +88,7 @@ int nxsem_trywait_slow(FAR sem_t *sem)
               break;
             }
 
-          new = nxsched_gettid();
+          new = NXSEM_MAKE_MHOLDER(nxsched_gettid());
         }
       else
         {
diff --git a/sched/semaphore/sem_wait.c b/sched/semaphore/sem_wait.c
index e4c065bfb47..a8391b13b9f 100644
--- a/sched/semaphore/sem_wait.c
+++ b/sched/semaphore/sem_wait.c
@@ -119,9 +119,13 @@ int nxsem_wait_slow(FAR sem_t *sem)
 
       mholder = atomic_fetch_or(NXSEM_MHOLDER(sem), NXSEM_MBLOCKING_BIT);
 
-      /* Avoid mutex recursion, which is not allowed. */
+      /* Avoid mutex recursion, which is not allowed.  The comparison uses
+       * the lock side's encoding so that ids of either sign compare the
+       * way they were stored.
+       */
 
-      DEBUGASSERT((mholder & (~NXSEM_MBLOCKING_BIT)) != nxsched_gettid());
+      DEBUGASSERT((mholder & (~NXSEM_MBLOCKING_BIT)) !=
+                  NXSEM_MAKE_MHOLDER(nxsched_gettid()));
 
       if (NXSEM_MACQUIRED(mholder))
         {