diff --git a/ULWatchItem.cs b/ULWatchItem.cs
index 0a4bacd..24ef231 100644
--- a/ULWatchItem.cs
+++ b/ULWatchItem.cs
@@ -309,7 +309,7 @@ namespace upper_limit_crawler
 		public float GetPrev5MASlope(int iTime)
 		{
 			int iLastIdx = m_1MinChart.IndexOfKey(iTime)-1;
-			if(iLastIdx <= 1)
+			if(iLastIdx <= 1 || iLastIdx >= m_5MAChart.Count)
 				return float.NaN;
 
 			MATick LastMA = m_5MAChart.Values[iLastIdx];