diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 5300126..67f6eac 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1900,6 +1900,9 @@
     }
 
 #if defined(MBEDTLS_THREADING_C)
+    /* If the lock operation fails we still wipe the slot.
+     * Operations will no longer work after a failed lock,
+     * but we still need to wipe the slot of confidential data. */
     mbedtls_mutex_lock(&mbedtls_threading_key_slot_mutex);
 #endif