diff --git a/src/common/pico_sync/include/pico/mutex.h b/src/common/pico_sync/include/pico/mutex.h
index c9b4c1b..4b5d175 100644
--- a/src/common/pico_sync/include/pico/mutex.h
+++ b/src/common/pico_sync/include/pico/mutex.h
@@ -31,8 +31,7 @@ extern "C" {
 
 typedef struct __packed_aligned mutex {
     lock_core_t core;
-    bool owned;
-    int8_t owner;
+    int8_t owner; //! core number or -1 for unowned
 } mutex_t;
 
 /*! \brief  Initialise a mutex structure
diff --git a/src/common/pico_sync/mutex.c b/src/common/pico_sync/mutex.c
index e2771d9..762a54d 100644
--- a/src/common/pico_sync/mutex.c
+++ b/src/common/pico_sync/mutex.c
@@ -13,6 +13,7 @@ static_assert(sizeof(mutex_t) == 8, "");
 
 void mutex_init(mutex_t *mtx) {
     lock_init(&mtx->core, next_striped_spin_lock_num());
+    mtx->owner = -1;
     __mem_fence_release();
 }
 
@@ -21,8 +22,7 @@ void __time_critical_func(mutex_enter_blocking)(mutex_t *mtx) {
     bool block = true;
     do {
         uint32_t save = spin_lock_blocking(mtx->core.spin_lock);
-        if (!mtx->owned) {
-            mtx->owned = true;
+        if (mtx->owner < 0) {
             mtx->owner = get_core_num();
             block = false;
         }
@@ -36,8 +36,7 @@ void __time_critical_func(mutex_enter_blocking)(mutex_t *mtx) {
 bool __time_critical_func(mutex_try_enter)(mutex_t *mtx, uint32_t *owner_out) {
     bool entered;
     uint32_t save = spin_lock_blocking(mtx->core.spin_lock);
-    if (!mtx->owned) {
-        mtx->owned = true;
+    if (mtx->owner < 0) {
         mtx->owner = get_core_num();
         entered = true;
     } else {
@@ -57,8 +56,7 @@ bool __time_critical_func(mutex_enter_block_until)(mutex_t *mtx, absolute_time_t
     bool block = true;
     do {
         uint32_t save = spin_lock_blocking(mtx->core.spin_lock);
-        if (!mtx->owned) {
-            mtx->owned = true;
+        if (mtx->owner < 0) {
             mtx->owner = get_core_num();
             block = false;
         }
@@ -75,10 +73,7 @@ bool __time_critical_func(mutex_enter_block_until)(mutex_t *mtx, absolute_time_t
 void __time_critical_func(mutex_exit)(mutex_t *mtx) {
     uint32_t save = spin_lock_blocking(mtx->core.spin_lock);
     assert(mtx->owned);
-    mtx->owned = 0;
-#ifndef NDEBUG
     mtx->owner = -1;
-#endif
     __sev();
     spin_unlock(mtx->core.spin_lock, save);
 }