diff --git a/library/hmac_drbg.c b/library/hmac_drbg.c
index 8bd8a05..876fa57 100644
--- a/library/hmac_drbg.c
+++ b/library/hmac_drbg.c
@@ -286,6 +286,7 @@
     if( ( ret = mbedtls_md_setup( &ctx->md_ctx, md_info, 1 ) ) != 0 )
         return( ret );
 
+    /* The mutex is initialized iff the md context is set up. */
 #if defined(MBEDTLS_THREADING_C)
     mbedtls_mutex_init( &ctx->mutex );
 #endif
@@ -457,6 +458,7 @@
         return;
 
 #if defined(MBEDTLS_THREADING_C)
+    /* The mutex is initialized iff the md context is set up. */
     if( ctx->md_ctx.md_info != NULL )
         mbedtls_mutex_free( &ctx->mutex );
 #endif