Subquery returned more than 1 value - asp.net

Question

I know this topic is all over the place, but I am not doing an INSERT, UPDATE, DELETE. My statement is a plain and simple SELECT statement and so far has worked with 116 different items in my database until I got to one.
I have a search engine and am going through every single product in our database to add information to it. This is all done through the website, but when I search for ProductID 331 and click on it, it goes to the error page that says Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.
It doesn't make any sense to me that the website would error with only this one product.
This is the statement I am using. Does anyone know why 1 product would be causing this error?
WebService:
Public Class ProductSearch
Inherits System.Web.Services.WebService
<WebMethod()> _
Public Function GetProducts(ByVal prefixText As String, ByVal count As Integer)
As String()
Dim ProductSql As String = "Select DISTINCT ProductID, ProductName
FROM Product WHERE ProductName
LIKE '%' & #prefixText & '%'
ORDER BY ProductName ASC"
Using sqlConn As New SqlConnection
(System.Configuration.ConfigurationManager.ConnectionStrings
("LocalSqlServer").ConnectionString)
sqlConn.Open()
Dim myCommand As New SqlCommand(ProductSql, sqlConn)
myCommand.Parameters.Add("#prefixText", SqlDbType.VarChar, 50)
.Value = prefixText
Dim myReader As SqlDataReader = myCommand.ExecuteReader()
Dim myTable As New DataTable
myTable.TableName = "ProductSearch"
myTable.Load(myReader)
sqlConn.Close()
Dim items As String() = New String(myTable.Rows.Count - 1) {}
Dim i As Integer = 0
For Each dr As DataRow In myTable.Rows
Dim id As String = dr("ProductID").ToString()
Dim name As String = dr("ProductName").ToString()
Dim item As String = AjaxControlToolkit.AutoCompleteExtender
.CreateAutoCompleteItem(name, id)
items.SetValue(item, i)
i += 1
Next
Return items
End Using
End Function
End Class
The aspx page that calls the webservice:
<%# Page Title="Product Search" Language="VB" MasterPageFile="~/MasterPage.master"
AutoEventWireup="false" CodeFile="Default.aspx.vb" Inherits="Default" %>
<%# Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit"
TagPrefix="asp" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
<script type="text/javascript">
function AutoCompleteClientMethod(source, eventArgs) {
var value = eventArgs.get_value();
window.location = ("/Product/Default.aspx?id=" + value)
}
</script>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="body" Runat="Server">
<asp:ScriptManager ID="ScriptManager1" runat="server">
<Services>
<asp:ServiceReference Path="ProductSearch.asmx" />
</Services>
</asp:ScriptManager>
<asp:TextBox ID="Search" runat="server" AutoComplete="off"></asp:TextBox>
<asp:AutoCompleteExtender ID="AutoCompleteExtender1" runat="server"
TargetControlID="Search" ServicePath="~/ProductSearch.asmx"
ServiceMethod="GetProducts" MinimumPrefixLength="1" CompletionSetCount="120"
EnableCaching="true" OnClientItemSelected="AutoCompleteClientMethod">
</asp:AutoCompleteExtender>
</div><!--End of main div -->
</asp:Content>
UPDATE: 11/9/2011 - I have found a couple more records that have this problem. They are ProductID 331-335. I have no idea what is going on here. Could it be that those products don't really exist or that they have some kind of bug?
Here is a list of ProductIDs and their corresponding ProductNames that have this error:
122 'Managed account section of the Web Site'
331 'Elliott Wave Principle Key to Market Behavior'
332 'Targeting Profitable Entry & Exit Points'
333 'Essentials of Trading It's not WHAT You Think, It's HOW You Think'
334 'Exceptional Trading The Mind Game'
335 'Fibonacci Analysis'

Answer 1

I assume this is the sub-select query, DISTINCT doesn't mean one result. You can use TOP 1 to guarantee one result, but it doesn't guarantee it is the one you want.
Select TOP 1 DISTINCT ProductID, ProductName
FROM Product WHERE ProductName
LIKE '%" & prefixText & "%'
ORDER BY ProductName ASC

Answer 2

Besides Rick's answer, I would add that you should never concatenate strings to form SQL statements. Use parametrized queries instead. String concatenation exposes you to SQL Injection attacks. Also, by using parametrized queries you may gain performance if the query plans can be reused.
See this other StackOverflow post for a good discussion regarding parametrized queries on VB.NET.

Answer 3

I figured out what the problem is. For some reason, these problematic products have more than one value assigned to them in data fields that SHOULD have only one item. The database has been changed recently so that doesn't happen, but I guess these 5 products were already messed up and have now been found out.
Thanks for all the help guys! I wish I would've thought to check further into the database sooner. (There are about 15 tables, so it's usually what I think of to do last)