diff --git a/src/6model/reprs/P6opaque.c b/src/6model/reprs/P6opaque.c
index a1beaaaed9..a5833eda30 100644
--- a/src/6model/reprs/P6opaque.c
+++ b/src/6model/reprs/P6opaque.c
@@ -718,7 +718,7 @@ static void compose(MVMThreadContext *tc, MVMSTable *st, MVMObject *info_hash) {
 
             /* Ensure we have a name. */
             if (MVM_is_null(tc, name_obj))
-                MVM_exception_throw_adhoc(tc, "P6opaque: missing attribute name for attribute %d", i);
+                MVM_exception_throw_adhoc(tc, "P6opaque: missing attribute name for attribute %"PRId64, i);
 
             if (REPR(name_obj)->ID == MVM_REPR_ID_MVMString) {
                 MVM_ASSIGN_REF(tc, &(st->header), name_map->names[i], (MVMString *)name_obj);
@@ -766,19 +766,19 @@ static void compose(MVMThreadContext *tc, MVMSTable *st, MVMObject *info_hash) {
                             case MVM_STORAGE_SPEC_BP_INT:
                                 if (repr_data->unbox_int_slot >= 0)
                                     MVM_exception_throw_adhoc(tc,
-                                        "Duplicate box_target for native int: attributes %d and %d", repr_data->unbox_int_slot, i);
+                                        "Duplicate box_target for native int: attributes %d and %"PRId64, repr_data->unbox_int_slot, i);
                                 repr_data->unbox_int_slot = cur_slot;
                                 break;
                             case MVM_STORAGE_SPEC_BP_NUM:
                                 if (repr_data->unbox_num_slot >= 0)
                                     MVM_exception_throw_adhoc(tc,
-                                        "Duplicate box_target for native num: attributes %d and %d", repr_data->unbox_num_slot, i);
+                                        "Duplicate box_target for native num: attributes %d and %"PRId64, repr_data->unbox_num_slot, i);
                                 repr_data->unbox_num_slot = cur_slot;
                                 break;
                             case MVM_STORAGE_SPEC_BP_STR:
                                 if (repr_data->unbox_str_slot >= 0)
                                     MVM_exception_throw_adhoc(tc,
-                                        "Duplicate box_target for native str: attributes %d and %d", repr_data->unbox_str_slot, i);
+                                        "Duplicate box_target for native str: attributes %d and %"PRId64, repr_data->unbox_str_slot, i);
                                 repr_data->unbox_str_slot = cur_slot;
                                 break;
                             default:
@@ -817,7 +817,7 @@ static void compose(MVMThreadContext *tc, MVMSTable *st, MVMObject *info_hash) {
             if (MVM_repr_exists_key(tc, attr_info, str_pos_del)) {
                 if (repr_data->pos_del_slot != -1)
                     MVM_exception_throw_adhoc(tc,
-                        "Duplicate positional delegate attributes: %d and "PRId64, repr_data->pos_del_slot, cur_slot);
+                        "Duplicate positional delegate attributes: %d and %"PRId64"", repr_data->pos_del_slot, cur_slot);
                 if (unboxed_type == MVM_STORAGE_SPEC_BP_NONE)
                     repr_data->pos_del_slot = cur_slot;
                 else
@@ -827,7 +827,7 @@ static void compose(MVMThreadContext *tc, MVMSTable *st, MVMObject *info_hash) {
             if (MVM_repr_exists_key(tc, attr_info, str_ass_del)) {
                 if (repr_data->ass_del_slot != -1)
                     MVM_exception_throw_adhoc(tc,
-                        "Duplicate associative delegate attributes: %d and "PRId64, repr_data->pos_del_slot, cur_slot);
+                        "Duplicate associative delegate attributes: %d and %"PRId64, repr_data->pos_del_slot, cur_slot);
                 if (unboxed_type == MVM_STORAGE_SPEC_BP_NONE)
                     repr_data->ass_del_slot = cur_slot;
                 else