"Run Keyword If" and setting a variable

The following code doesn't work. I want to set a variable if a condition is true.

    Run Keyword If    ${MAC} == 110299160221421    ${device_serial}=    set variable    ${device_serial_1}

I get the error:

No keyword with name '=' found

This is working but doesn't change a variable.

    Run Keyword If    ${MAC} == 110299160221421    Log to console    111