chrome.runtime.onMessage is not catching messages when the extension loads

I'm not sure why, but it seems like chrome.runtime.onMessage is not loading when my extension loads. I tried to trouble shoot with a basic message:

popup.js

chrome.runtime.onMessage.addListener(function(request, sender, sendResponse) {
    if (request.contentMessage == "From content script") {
        sendResponse({popupMessage:"Message received"});
    }
});

content.js

chrome.runtime.sendMessage({contentMessage:"From content script"}, function(response) {
    console.log(response);
}

When I run the extension, the error log for content.js shows that it is not receiving a response: "Error in event handler for (unknown): TypeError: Cannot read property 'popupMessage' of undefined.

My popup.js log shows nothing.

If I open up my popup.js console and manually paste the code and then try re-sending my message from content.js (by just attaching it to a button), then everything works.

Can anyone help me figure out why my popup.js script is not catching the message and returning a response?