diff --git a/mongoose.c b/mongoose.c
index 9cfd1427..3d7d715d 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -293,6 +293,7 @@ static void dns_cb(struct mg_connection *c, int ev, void *ev_data,
   } else if (ev == MG_EV_CLOSE) {
     for (d = s_reqs; d != NULL; d = tmp) {
       tmp = d->next;
+      mg_error(d->c, "DNS error");
       mg_dns_free(d);
     }
   }
@@ -3232,7 +3233,9 @@ void mg_connect_resolved(struct mg_connection *c) {
 struct mg_connection *mg_connect(struct mg_mgr *mgr, const char *url,
                                  mg_event_handler_t fn, void *fn_data) {
   struct mg_connection *c = NULL;
-  if ((c = alloc_conn(mgr, 1, INVALID_SOCKET)) == NULL) {
+  if (url == NULL || url[0] == '\0') {
+    LOG(LL_ERROR, ("null url"));
+  } else if ((c = alloc_conn(mgr, 1, INVALID_SOCKET)) == NULL) {
     LOG(LL_ERROR, ("OOM"));
   } else {
     LIST_ADD_HEAD(struct mg_connection, &mgr->conns, c);
diff --git a/src/dns.c b/src/dns.c
index 7eace2e2..c9dde492 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -183,6 +183,7 @@ static void dns_cb(struct mg_connection *c, int ev, void *ev_data,
   } else if (ev == MG_EV_CLOSE) {
     for (d = s_reqs; d != NULL; d = tmp) {
       tmp = d->next;
+      mg_error(d->c, "DNS error");
       mg_dns_free(d);
     }
   }
diff --git a/src/sock.c b/src/sock.c
index d13a22c9..43ef27a5 100644
--- a/src/sock.c
+++ b/src/sock.c
@@ -361,7 +361,9 @@ void mg_connect_resolved(struct mg_connection *c) {
 struct mg_connection *mg_connect(struct mg_mgr *mgr, const char *url,
                                  mg_event_handler_t fn, void *fn_data) {
   struct mg_connection *c = NULL;
-  if ((c = alloc_conn(mgr, 1, INVALID_SOCKET)) == NULL) {
+  if (url == NULL || url[0] == '\0') {
+    LOG(LL_ERROR, ("null url"));
+  } else if ((c = alloc_conn(mgr, 1, INVALID_SOCKET)) == NULL) {
     LOG(LL_ERROR, ("OOM"));
   } else {
     LIST_ADD_HEAD(struct mg_connection, &mgr->conns, c);
diff --git a/test/unit_test.c b/test/unit_test.c
index d98d3e00..b41f3a74 100644
--- a/test/unit_test.c
+++ b/test/unit_test.c
@@ -1152,6 +1152,23 @@ static void fn1(struct mg_connection *c, int ev, void *ev_data, void *fn_data) {
   (void) c;
 }
 
+static void test_dns_timeout(const char *dns_server_url, const char *errstr) {
+  // Test timeout
+  struct mg_mgr mgr;
+  char buf[100] = "";
+  int i;
+  mg_mgr_init(&mgr);
+  mgr.dns4.url = dns_server_url;
+  mgr.dnstimeout = 10;
+  LOG(LL_DEBUG, ("opening dummy DNS listener..."));
+  mg_listen(&mgr, mgr.dns4.url, NULL, NULL);  // Just discard our queries
+  mg_http_connect(&mgr, "http://google.com", fn1, buf);
+  for (i = 0; i < 50 && buf[0] == '\0'; i++) mg_mgr_poll(&mgr, 1);
+  mg_mgr_free(&mgr);
+  LOG(LL_DEBUG, ("buf: [%s]", buf));
+  ASSERT(strcmp(buf, errstr) == 0);
+}
+
 static void test_dns(void) {
   struct mg_dns_message dm;
   //       txid  flags numQ  numA  numAP numOP
@@ -1171,21 +1188,9 @@ static void test_dns(void) {
   ASSERT(mg_dns_parse(data, sizeof(data), &dm) == 1);
   ASSERT(strcmp(dm.name, "") == 0);
 
-  {
-    // Test timeout
-    struct mg_mgr mgr;
-    char buf[100] = "";
-    int i;
-    mg_mgr_init(&mgr);
-    mgr.dns4.url = "udp://127.0.0.1:12345";
-    mgr.dnstimeout = 10;
-    LOG(LL_DEBUG, ("opening dummy DNS listener..."));
-    mg_listen(&mgr, mgr.dns4.url, NULL, NULL);  // Just discard our queries
-    mg_http_connect(&mgr, "http://google.com", fn1, buf);
-    for (i = 0; i < 50 && buf[0] == '\0'; i++) mg_mgr_poll(&mgr, 1);
-    mg_mgr_free(&mgr);
-    ASSERT(strcmp(buf, "DNS timeout") == 0);
-  }
+  test_dns_timeout("udp://127.0.0.1:12345", "DNS timeout");
+  test_dns_timeout("", "resolver");
+  test_dns_timeout("tcp://0.0.0.0:0", "DNS error");
 }
 
 static void test_util(void) {