diff --git a/lib/krb5/keytab.c b/lib/krb5/keytab.c
index 382c0d19d..87b6f137c 100644
--- a/lib/krb5/keytab.c
+++ b/lib/krb5/keytab.c
@@ -305,6 +305,8 @@ krb5_kt_start_seq_get(krb5_context context,
 		      krb5_keytab id,
 		      krb5_kt_cursor *cursor)
 {
+    if(id->start_seq_get == NULL)
+	return HEIM_ERR_OPNOTSUPP;
     return (*id->start_seq_get)(context, id, cursor);
 }
 
@@ -314,6 +316,8 @@ krb5_kt_next_entry(krb5_context context,
 		   krb5_keytab_entry *entry,
 		   krb5_kt_cursor *cursor)
 {
+    if(id->next_entry)
+	return HEIM_ERR_OPNOTSUPP;
     return (*id->next_entry)(context, id, entry, cursor);
 }
 
@@ -323,6 +327,8 @@ krb5_kt_end_seq_get(krb5_context context,
 		    krb5_keytab id,
 		    krb5_kt_cursor *cursor)
 {
+    if(id->end_seq_get)
+	return HEIM_ERR_OPNOTSUPP;
     return (*id->end_seq_get)(context, id, cursor);
 }