Invalid icon_url values intermittently returned

Calls to retrieve data for a specific location are intermittently returned incorrect values for the icon_url field.  Please see the following JSON snippet from data returned by the call that contains icon_url values that end in 'nt_.gif': https://api.wunderground.com/api/<key>/conditions/forecast/q/33626.json <br><br>(Note that subsequent calls to this same URL just a few minutes later returned data with correct icon_url values in it)<br>

<br>"forecast":{"txt_forecast": {
		"date":"",

		"forecastday": [
		{
		"period":0,
		"icon":"partlycloudy",
		"icon_url":"http://icons.wxug.com/i/c/k/partlycloudy.gif",
		"title":"",
		"fcttext":"",
		"fcttext_metric":"",
		"pop":"0"
		}
		,
		{
		"period":1,
		"icon":"nt_",
		"icon_url":"http://icons.wxug.com/i/c/k/nt_.gif",
		"title":"",
		"fcttext":"",
		"fcttext_metric":"",
		"pop":"0"
		}
		,
		{
		"period":2,
		"icon":"mostlycloudy",
		"icon_url":"http://icons.wxug.com/i/c/k/mostlycloudy.gif",
		"title":"",
		"fcttext":"",
		"fcttext_metric":"",
		"pop":"0"
		}
		,
		{
		"period":3,
		"icon":"nt_",
		"icon_url":"http://icons.wxug.com/i/c/k/nt_.gif",
		"title":"",
		"fcttext":"",
		"fcttext_metric":"",
		"pop":"0"
		}
		,
		{
		"period":4,
		"icon":"mostlycloudy",
		"icon_url":"http://icons.wxug.com/i/c/k/mostlycloudy.gif",
		"title":"",
		"fcttext":"",
		"fcttext_metric":"",
		"pop":"2"
		}
		,
		{
		"period":5,
		"icon":"nt_",
		"icon_url":"http://icons.wxug.com/i/c/k/nt_.gif",
		"title":"",
		"fcttext":"",
		"fcttext_metric":"",
		"pop":"0"
		}
		,
		{
		"period":6,
		"icon":"chancerain",
		"icon_url":"http://icons.wxug.com/i/c/k/chancerain.gif",
		"title":"",
		"fcttext":"",
		"fcttext_metric":"",
		"pop":"24"
		}
		,
		{
		"period":7,
		"icon":"nt_",
		"icon_url":"http://icons.wxug.com/i/c/k/nt_.gif",
		"title":"",
		"fcttext":"",
		"fcttext_metric":"",
		"pop":"0"
		}
		]
		},...