diff --git a/libasn1fix/asn1fix_retrieve.h b/libasn1fix/asn1fix_retrieve.h
index 2d529ef..95b8330 100644
--- a/libasn1fix/asn1fix_retrieve.h
+++ b/libasn1fix/asn1fix_retrieve.h
@@ -31,7 +31,7 @@
  */
 asn1p_module_t *asn1f_lookup_module(arg_t *arg,
 		const char *module_name,
-		asn1p_oid_t *module_oid);
+		const asn1p_oid_t *module_oid);
 
 /*
  * Return the reference to a destination of the given reference,
@@ -40,7 +40,7 @@
 asn1p_expr_t *asn1f_lookup_symbol(arg_t *arg,
 		asn1p_module_t *mod,
 		asn1p_expr_t *rhs_pspecs,
-		asn1p_ref_t *ref);
+		const asn1p_ref_t *ref);
 
 /*
  * Recursively find the original type for the given expression.