Cannot unzip zip file downloaded with Powershell - powershell

Question

I have a zip file hosted at a URL. I'm writing a Powershell script to get that zip file using Invoke-RestMethod and save it to disk. When I run the Powershell script, the zip file is downloaded and saved to disk, however, I cannot open or extract it. Does anyone know what I'm doing wrong? Here is my code that handles the zip download/save. Here is my code (I've changed the URL for the zip for security reasons. If I type in the actual URL into a browser it will be downloaded by the browser after which I can locate it on disk and extract it).
$username = "uname"
$password = "pword"
$boundary = [guid]::NewGuid().ToString().Replace("-", "").Substring(0, 16)
$enc = [System.Text.Encoding]::GetEncoding("utf-8")
$base64AuthInfo = [Convert]::ToBase64String([Text.Encoding]::ASCII.GetBytes(("{0}:{1}" -f $username,$password)))
$headers = #{}
$headers.Add("Host", "api.bitbucket.org")
$headers.Add("Authorization", "Basic $base64AuthInfo")
$repoResult
$repoResult = Invoke-WebRequest -Uri "https://bitbucket.org/teamAccount/repoName/get/master.zip" -Headers $headers -Method GET -Verbose -OutFile .\output.zip
}
EDIT/UPDATE:
Thanks to TesselatedHeckler's comment, I've discovered that the downloaded zip is actually an HTML login page. I also realized that I didn't have my headers (which contains the login info) in Invoke-RestMethod line. I've updated the post and my script. Now, whenever I run my script, I get a (500) Internal Server Error.