{"String was not recognized as a valid timespan."}

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C#

I am trying to parse string to timespan but i m getting the error {"String was not recognized as a valid TimeSpan."}
My code is:
public string timeconvert(string grace, string difftime)
{
string totalTime;
TimeSpan t1 = TimeSpan.Parse(difftime);// here come the error
//In difftime value comes from function like 9.30 ,i thinks that's y it showing error.

if (string.IsNullOrEmpty(grace))
grace = "0";

TimeSpan g1 = TimeSpan.Parse(grace);
TimeSpan TotalTime = t1 + g1;
totalTime = TotalTime.ToString();


return totalTime;
}
// i m calling this function
item.ATT_TOTAL_ATT = timeconvert(Convert.ToString(item.ATT_BIOIN_GRACE), (Convert.ToString(item.ATT_DIFF_TIME)));

What I have tried:

{"String was not recognized as a valid TimeSpan."}