diff --git a/ChangeLog b/ChangeLog
index 60383a9..8090a70 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -29,6 +29,8 @@
      a contribution from Tobias Tangemann. #541
    * Fixed cert_app sample program for debug output and for use when no root
      certificates are provided.
+   * Fixed default threading implementation to avoid accidental double
+     initialisations and double frees.
 
 Changes
    * Extended test coverage of special cases, and added new timing test suite.
diff --git a/library/threading.c b/library/threading.c
index 1b6d9cd..83ec01a 100644
--- a/library/threading.c
+++ b/library/threading.c
@@ -32,7 +32,7 @@
 #if defined(MBEDTLS_THREADING_PTHREAD)
 static void threading_mutex_init_pthread( mbedtls_threading_mutex_t *mutex )
 {
-    if( mutex == NULL )
+    if( mutex == NULL || mutex->is_valid )
         return;
 
     mutex->is_valid = pthread_mutex_init( &mutex->mutex, NULL ) == 0;
@@ -40,10 +40,11 @@
 
 static void threading_mutex_free_pthread( mbedtls_threading_mutex_t *mutex )
 {
-    if( mutex == NULL )
+    if( mutex == NULL || !mutex->is_valid )
         return;
 
     (void) pthread_mutex_destroy( &mutex->mutex );
+    mutex->is_valid = 0;
 }
 
 static int threading_mutex_lock_pthread( mbedtls_threading_mutex_t *mutex )