I can't work out what's wrong with this CSS:

PHP Code:
#right-panel ol {
    
padding0px 0px 0px 2em;
    
text-indent0em;
    
margin:_0px;

The w3c validator gives this error:

Line: 76 Context : #right-panel ol

Invalid number : margin 0px is not a margin-top value : 0px
I'm not understanding this. Setting the top margin to 1 etc doesn't do any good. What am I missing?