Linq – SharePoint, List.Items and List.GetItems(Query) and Linq

Following on from suggestions, I am trying to use List.GetItems(Query) to retrieve my initial data subset rather than the entire list contents via List.Items. However, whereas List.Items.Cast() results in a usable IEnumerable for Linq, List.GetItems(Query).Cast() does not.

Working Code:

IEnumerable<SPListItem> results = SPContext.Current.Web.Lists[ListName].Items.Cast<SPListItem>().Where(item => item["Date"] != null).Where(item => DateTime.Parse(item["Date"].ToString()) >= StartDate).Where(item => DateTime.Parse(item["Date"].ToString()) <= EndDate);
MessageLine = results.Count().ToString();

Non-working Code:

string SPStartDate = SPUtility.CreateISO8601DateTimeFromSystemDateTime(this.StartDate);
string SPEndDate =   SPUtility.CreateISO8601DateTimeFromSystemDateTime(this.EndDate);

SPQuery MyQuery = new SPQuery();
MyQuery.Query = "<Where><And><And><Geq><FieldRef Name='Date'/><Value Type='DateTime'>" + SPStartDate + "</Value></Geq><Leq><FieldRef Name='Date'/><Value Type='DateTime'>" + SPEndDate + "</Value></Leq></And></Where>";

IEnumerable<SPListItem> results = SPContext.Current.Web.Lists[ListName].GetItems(MyQuery).Cast<SPListItem>();

MessageLine = results.Count().ToString();

The List.GetItems(Query).Cast() method produces the following Exception on the .Count() line:

Microsoft.SharePoint.SPException:
Cannot complete this action. Please
try again. —>
System.Runtime.InteropServices.COMException
(0x80004005): Cannot complete this
action. Please try again. at
Microsoft.SharePoint.Library.SPRequestInternalClass.GetListItemDataWithCallback(String
bstrUrl, String bstrListName, String
bstrViewName, String bstrViewXml,
SAFEARRAYFLAGS fSafeArrayFlags,
ISP2DSafeArrayWriter pSACallback,
ISPDataCallback pPagingCallback,
ISPDataCallback pSchemaCallback) at
Microsoft.SharePoint.Library.SPRequest.GetListItemDataWithCallback(String
bstrUrl, String bstrListName, String
bstrViewName, String bstrViewXml,
SAFEARRAYFLAGS fSafeArrayFlags,
ISP2DSafeArrayWriter pSACallback,
ISPDataCallback pPagingCallback,
ISPDataCallback pSchemaCallback) —
End of inner exception stack trace —
at
Microsoft.SharePoint.Library.SPRequest.GetListItemDataWithCallback(String
bstrUrl, String bstrListName, String
bstrViewName, String bstrViewXml,
SAFEARRAYFLAGS fSafeArrayFlags,
ISP2DSafeArrayWriter pSACallback,
ISPDataCallback pPagingCallback,
ISPDataCallback pSchemaCallback) at
Microsoft.SharePoint.SPListItemCollection.EnsureListItemsData()
at
Microsoft.SharePoint.SPListItemCollection.Undirty()
at
Microsoft.SharePoint.SPBaseCollection.System.Collections.IEnumerable.GetEnumerator()
at
System.Linq.Enumerable.d__aa1.MoveNext()
at
System.Linq.Enumerable.Count[TSource](IEnumerable1
source) at
Test.GetTransactionsInPeriod() at
Test.CreateChildControls()

Can anyone suggest anything?