diff --git a/library/psa_crypto_se.c b/library/psa_crypto_se.c
index 523c621..2cda4cc 100644
--- a/library/psa_crypto_se.c
+++ b/library/psa_crypto_se.c
@@ -225,6 +225,12 @@
                                          attributes, method,
                                          *slot_number );
     }
+    else if( method == PSA_KEY_CREATION_REGISTER )
+    {
+        /* The application didn't specify a slot number. This doesn't
+         * make sense when registering a slot. */
+        return( PSA_ERROR_INVALID_ARGUMENT );
+    }
     else
     {
         /* The application didn't tell us which slot to use. Let the driver