Trouble with 'Error no 3 while sending order'

Hello,

I have an EA which calculates Lots size. I am trying to change it to send orders for a Fixed size. I have added the following:

extern double DefaultLots =0.5;

to the input parameters section at the start of the code.

I then set DefaultLots and call the order send function like this:

lots =DefaultLots;
ticket1=OrderSendCustom(
symbol,
OP_SELLLIMIT,
lots,
Entry,
Slippage,
SL,
TP,
"0",
Magic,expirationTime);

but when I try to place the script order I get "Error no 3 while sending order".

Can anyone tell me what I'm doing wrong? I haven't been able to contact the original coder to get him to change this yet, so am trying myself.

thanks,

Erron