diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 7591b6b..4677ea9 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -3658,13 +3658,13 @@
         case PSA_ALG_CHACHA20_POLY1305:
             if( nonce_length == 12 )
                 return( PSA_SUCCESS );
-            break;
+            return( PSA_ERROR_NOT_SUPPORTED );
 #endif /* PSA_WANT_ALG_CHACHA20_POLY1305 */
         default:
-            break;
+            return( PSA_ERROR_NOT_SUPPORTED );
     }
 
-    return( PSA_ERROR_NOT_SUPPORTED );
+    return( PSA_ERROR_INVALID_ARGUMENT );
 }
 
 psa_status_t psa_aead_encrypt( mbedtls_svc_key_id_t key,