diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index a2f6897..ca461c2 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1510,10 +1510,6 @@
 {
     int ret = 0;
     psa_status_t status = PSA_SUCCESS;
-    if( ! operation->key_set )
-        return( PSA_ERROR_BAD_STATE );
-    if( operation->iv_required && ! operation->iv_set )
-        return( PSA_ERROR_BAD_STATE );
 
     /* Fill the output buffer with something that isn't a valid mac
      * (barring an attack on the mac and deliberately-crafted input),
@@ -1524,6 +1520,11 @@
     if( mac_size != 0 )
         memset( mac, '!', mac_size );
 
+    if( ! operation->key_set )
+        return( PSA_ERROR_BAD_STATE );
+    if( operation->iv_required && ! operation->iv_set )
+        return( PSA_ERROR_BAD_STATE );
+
     if( mac_size < operation->mac_size )
         return( PSA_ERROR_BUFFER_TOO_SMALL );