Linker not working on solc v0.4.23

Question

I can't seem to make the linker.linkBytecode() method to work...

Here's how I'm using it:

const linkedBytecode = linker.linkBytecode(mainContract.bytecode, {
        'UserLib': userLibContract.options.address
 });

If I try to console.log(linkedBytecode) it outputs the bytecode with placeholders that should've been replaced by the linker!

I noticed that the compiled bytecode of my mainContract outputs something like ...5173__IssueLib.sol:IssueLib_________________916.... whereas on the official documentation it says that it should output something like ...5173__IssueLib_______________916....

What Am I missing here?

Thanks in advance! :)