How can I resolve this problem when doing a metachecker

See more:

Metafile

Hello,

So I made meta tags for individual posts. I saw they didn't change when searched for in google. When I did metachecker it gave the following error.

Can someone help me with this?
I would be very grateful

Server Error in '/' Application.
Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:


Line 213: {
Line 214: Metatag cur = new Metatag();
Line 215: cur.Content = m.Attributes["content"].Value;
Line 216: if (m.Attributes["http-equiv"] != null)
Line 217: {


Source File: d:\www\metachecker.net\Site\Analise.aspx.cs Line: 215

Stack Trace:


[NullReferenceException: Object reference not set to an instance of an object.]
Analise.AnaliseMetatags(XhtmlDocument document) in d:\www\metachecker.net\Site\Analise.aspx.cs:215
Analise.Page_Load(Object sender, EventArgs e) in d:\www\metachecker.net\Site\Analise.aspx.cs:112
System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +24
System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +41
System.Web.UI.Control.OnLoad(EventArgs e) +131
System.Web.UI.Control.LoadRecursive() +65
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2427

What I have tried:

I have tried to find the error in my html, unsuccessfully...