Any idea what could be causing this?

Input string was not in a correct format.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.FormatException: Input string was not in a correct format.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace: 

[FormatException: Input string was not in a correct format.]

   System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal) +7471623

   System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info) +119

   Umbraco.AutoComplete.DataEditor.Save() +320

   umbraco.controls.ContentControl.saveClick(Object Sender, ImageClickEventArgs e) in d:\TeamCity\buildAgent\work\7380c184e9fcd3ea\umbraco\presentation\umbraco\controls\ContentControl.cs:158

   System.Web.UI.WebControls.ImageButton.OnClick(ImageClickEventArgs e) +108

   System.Web.UI.WebControls.ImageButton.RaisePostBackEvent(String eventArgument) +118

   System.Web.UI.WebControls.ImageButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10

   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13

   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +36

   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1565

Version Information: Microsoft .NET Framework Version:2.0.50727.4927; ASP.NET Version:2.0.50727.4927