diff --git a/library/asn1write.c b/library/asn1write.c
index e50c17c..4ea8a46 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -118,7 +118,7 @@
     return( len );
 }
 
-int asn1_write_oid( unsigned char **p, unsigned char *start, char *oid )
+int asn1_write_oid( unsigned char **p, unsigned char *start, const char *oid )
 {
     int ret;
     size_t len = 0;
@@ -140,7 +140,7 @@
 }
 
 int asn1_write_algorithm_identifier( unsigned char **p, unsigned char *start,
-                                     char *algorithm_oid )
+                                     const char *algorithm_oid )
 {
     int ret;
     size_t null_len = 0;