diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c
index 64b536a..e275f1a 100644
--- a/library/ctr_drbg.c
+++ b/library/ctr_drbg.c
@@ -99,6 +99,7 @@
         return;
 
 #if defined(MBEDTLS_THREADING_C)
+    /* The mutex is initialized iff f_entropy is set. */
     if( ctx->f_entropy != NULL )
         mbedtls_mutex_free( &ctx->mutex );
 #endif
@@ -376,6 +377,7 @@
 
     memset( key, 0, MBEDTLS_CTR_DRBG_KEYSIZE );
 
+    /* The mutex is initialized iff f_entropy is set. */
 #if defined(MBEDTLS_THREADING_C)
     mbedtls_mutex_init( &ctx->mutex );
 #endif