diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 6c43158..1123a78 100755
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1491,6 +1491,7 @@
     size_t key_bits;
     unsigned char tag[16];
     mbedtls_cipher_id_t cipher_id;
+    *ciphertext_length = 0;
 
     if( ciphertext_size < ( plaintext_length + sizeof( tag ) ) )
         return( PSA_ERROR_INVALID_ARGUMENT );