diff --git a/EonaCat.DnsTester/MainForm.cs b/EonaCat.DnsTester/MainForm.cs
index f00329d..fbbde27 100644
--- a/EonaCat.DnsTester/MainForm.cs
+++ b/EonaCat.DnsTester/MainForm.cs
@@ -37,7 +37,7 @@ namespace EonaCat.DnsTester
                 }
             }
 
-            if (_useCustomDnsServers && (!chkDns1.Checked || chkDns2.Checked))
+            if (_useCustomDnsServers && (!chkDns1.Checked && !chkDns2.Checked))
             {
                 MessageBox.Show("Please enable DNS 1 or 2 before using custom Dns");
                 return;
@@ -50,6 +50,11 @@ namespace EonaCat.DnsTester
             int numThreads = (int)numericUpDown2.Value; // number of concurrent threads to use
             int maxUrls = (int)numericUpDown1.Value; // maximum number of unique URLs to retrieve
             int numUrlsPerThread = maxUrls / numThreads;
+            if (numUrlsPerThread == 0)
+            {
+                numUrlsPerThread = maxUrls;
+                numThreads = 1;
+            }
 
 
             urls = await UrlHelper.RetrieveUrls(numThreads, numUrlsPerThread);