Why am I getting this error: The ConnectionString property has not been initialized

Why am I getting this error: The ConnectionString property has not been
initialized

I have searched and tried everything but can't figure this one out. I am
trying to do something simple but it seems as though I am doing something
wrong. Basically, any user that has made a deposit, I want to return true,
if they have not, I want to return false. This should be easy I suppose
but I am stumped on this.
Here is the error:
The ConnectionString property has not been initialized.
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.InvalidOperationException: The ConnectionString
property has not been initialized.
Source Error:
Line 58: Line 59: Cmd.Parameters.AddWithValue("@UserID", userId); Line 60:
con.Open(); Line 61: Line 62: result = (int)Cmd.ExecuteScalar();
Here is the top of the stack trace:
[InvalidOperationException: The ConnectionString property has not been
initialized.] System.Data.SqlClient.SqlConnection.PermissionDemand()
+4879939
System.Data.SqlClient.SqlConnectionFactory.PermissionDemand(DbConnection
outerConnection) +20
System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection
outerConnection, DbConnectionFactory connectionFactory) +117
System.Data.SqlClient.SqlConnection.Open() +122
Here is my method for returning true or false:
public static bool HasDeposit(int userId)
{
int result = 0;
//since executeScalar is intended to retreive only a single value
//from a query, we select the number of results instead of the email
address
//of each matching result.
string queryTransaction = "SELECT COUNT(UserID) FROM Transaction WHERE
TransactionTypeID = 6 AND UserID = @UserID";
string constr =
System.Configuration.ConfigurationManager.AppSettings["ConnectionString"];
SqlConnection con = new SqlConnection(constr);
SqlCommand Cmd = new SqlCommand(queryTransaction, con);
Cmd.Parameters.AddWithValue("@UserID", userId);
con.Open();
result = (int)Cmd.ExecuteScalar();
//returning a boolean comparator works like this :
//will return true if the result is greater than zero, but false if it
is not.
con.Close();
return result > 0;
}
Any help / guidance on this would be much appreciated.