In the page "if then else" on mirc's helpfile it says:
The ! not prefix
You can use the ! prefix in front of variables and identifiers in an if-then-else statement to negate a value. The following statements are equivalent.

if (%x == $null) echo x has no value

if (!%x) echo x has no value

which is wrong, this is not a major problem, but should be fixed in next version, since it might get some people confused.