diff --git a/libasn1parser/asn1p_constr.h b/libasn1parser/asn1p_constr.h
index 7324605..7edd63f 100644
--- a/libasn1parser/asn1p_constr.h
+++ b/libasn1parser/asn1p_constr.h
@@ -69,7 +69,7 @@
 } asn1p_constraint_t;
 
 /* Human-readable constraint type description */
-char *asn1p_constraint_type2str(enum asn1p_constraint_type_e);
+const char *asn1p_constraint_type2str(enum asn1p_constraint_type_e);
 
 /*
  * Constructors and a recursive destructor.