Disable a cell when A cell is blank

I want to create a google spreadsheet where I have 2 columns involve. The scenario is:

in my A column, I can input and column B has a drop down. What I want to do is disable column B unless column A has value.

Column A1 has value Column B1 is enabled Column A2 has no value Column B2 is disabled and so on 

I created a custom formula:

= isblank(A1) 

Image is:

enter image description here