Selecting element from drop down menu doesn't work - Selenium IDE

I'm trying to select an element that contains a partial text from a drop down menu. e.g. If I have an element like "12 streetname town" in a drop down menu and I try to select it like this:

<td>select</td>
<td>//option[contains(text(),'12')]</td>
<td></td>

or:

<td>select</td>
<td>//*[contains(@id,'my_id')]/option[contains(text(),'12')]
<td></td>

I get:

[error] Specified element is not a Select (has no options)

but If I select the element like this:

<td>select</td>
<td>//*[contains(@id,'my_id')]</td>
<td>value=12 streenname town</td> 

it works.

So how can I select an element only by partial text?

Thanks!