Getting "is not a thenable" message while using "eventually" in protractor chai

When I tried to verify the condition as below.

 var val1 = "ONE";
 var val2 = "TWO";
 expect(val1==val2).to.eventually.equal(false)

I'm getting false is not a thenable message, If I removed eventually condition as below then it working fine.

 var val1 = "ONE";
 var val2 = "TWO";
 expect(val1==val2).to.equal(false)

Can anyone help me to understand the difference. Also If the condition fails, It displays the error message and not executing the hooks.js.