c# - Specified cast is not valid
<p>I'm trying to code for a project, but the non-valid specific cast error keeps coming out. Can anyone help me as I am stumped. Thanks in advance.</p>

<pre><code>Server Error in '/c#project' Application.

Specified cast is not valid.

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.InvalidCastException: Specified cast is not valid.

Source Error: 


Line 39:         cmd.Parameters.Add("@ProductId", OleDbType.Char).Value = strProductId;
Line 40:         object oQty = cmd.ExecuteScalar();
Line 41:         int intQuantityOnHand = (int)oQty;
Line 42:         mDB.Close();
Line 43:         int intBuyQuantity = int.Parse(ddlQty.Items[ddlQty.SelectedIndex].ToString());

Source File: c:\Users\jacob\Desktop\c#project\ProductDetails.aspx.cs    Line: 41 

Stack Trace: 


[InvalidCastException: Specified cast is not valid.]
   ProductDetails.btnBuy_Click(Object sender, EventArgs e) in c:\Users\jacob\Desktop\c#project\ProductDetails.aspx.cs:41
   System.Web.UI.WebControls.Button.OnClick(EventArgs e) +118
   System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +112
   System.Web.UI.WebControls.Button.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) +5563

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.272
</code></pre>
