error: in create wiki page programmaticallu in shrepoint 2013

hi,

here is my code.

 using (SPSite site = new SPSite(strSiteUrl))
{
 SPWeb currentWeb= site.RootWeb;
 SPList spList = currentWeb.Lists[strlibraryname];//GetList(strSiteUrl + "/" + strlibraryname + "/forms/allpages.aspx");
  SPFolder wikiRootFolder = spList.RootFolder;
 if (spList != null) {
 //try 
 {
  SPFile wikiPage = wikiRootFolder.Files.Add(String.Format("{0}/{1}", wikiRootFolder,"brtestcode.aspx"),SPTemplateFileType.WikiPage);
   SPListItem wikiItem = wikiPage.Item;
   wikiItem[SPBuiltInFieldId.WikiField] = "Wiki Page Content "; 
 wikiItem.UpdateOverwriteVersion();
 wikiRootFolder.Update();
 }
}
}

while create wiki p age it is throw error like "targetFilePath must be a server relative URL. Parameter name: targetFilePath "

in the below line:

SPFile wikiPage = wikiRootFolder.Files.Add(String.Format("{0}/{1}", wikiRootFolder,"brtestcode.aspx"),SPTemplateFileType.WikiPage)

then i have added "/" in front of {0}

SPFile wikiPage = wikiRootFolder.Files.Add(String.Format("/{0}/{1}", wikiRootFolder,"brtestcode.aspx"),SPTemplateFileType.WikiPage)

again it is throw error like" Value does not fall within the expected range." in the above line of code.