diff --git a/smmapdfw/libsmmapdfw/htdns.c b/smmapdfw/libsmmapdfw/htdns.c
index e1a6b9d..cf1f6eb 100644
--- a/smmapdfw/libsmmapdfw/htdns.c
+++ b/smmapdfw/libsmmapdfw/htdns.c
@@ -350,10 +350,25 @@ static void** get_rrs(char *domain, int qtype) {
 }
 
 mx_rdata_t** get_mx_rrs(char *domain) {
-  return (mx_rdata_t**) get_rrs(domain, T_MX);
+  if (domain == NULL)
+    return NULL;
+
+  rdata_t **rdata = get_rrs(domain, T_MX);
+
+  if ((NULL == rdata) || (NULL == *rdata)) {
+    return NULL;
+  } else if ((*rdata)->type != T_MX) {
+    free_rrs((void**)rdata);
+    return NULL;
+  } else {
+    return (mx_rdata_t**) rdata;
+  }
 }
 
 a_rdata_t** get_a_rrs(char *domain) {
+  if (domain == NULL)
+    return NULL;
+
   rdata_t **rdata, **rdata2;
   cname_rdata_t **cname_rdata;
 
@@ -376,6 +391,9 @@ a_rdata_t** get_a_rrs(char *domain) {
 
 #define min(A,B) ((A<B) ? A : B)
 mx_rdata_t** get_best_mx_rrs(char *domain) {
+  if (domain == NULL)
+    return NULL;
+
   mx_rdata_t **all_mx_rrs, **mx_rdata2, **best_mx_rrs;
   int min_pref = 10000000;
   int all_cnt = 0;