azure-pipelines-tasks: Exception in PublishSymbols (Exception calling "Filter" with "3" argument(s): "Value cannot be null)

I am trying to use the publish symbols task. The following is my task configuration:

  - task: PublishSymbols@2
    displayName: publish symbols
    inputs:
      SearchPattern: ___output/**/*.pdb
      SymbolServerType: TeamServices

I keep getting the error Exception calling “Filter” with “3” argument(s): "Value cannot be null.

I have pasted the debug output below. What am I missing to get this task working?

##[debug]Evaluating condition for step: 'publish symbols'
##[debug]Evaluating: SucceededNode()
##[debug]Evaluating SucceededNode:
##[debug]=> True
##[debug]Result: True
##[section]Starting: publish symbols
==============================================================================
Task         : Index Sources & Publish Symbols
Description  : Index your source code and publish symbols to a file share or Azure Artifacts Symbol Server
Version      : 2.0.14
Author       : Microsoft Corporation
Help         : See [more information](https://go.microsoft.com/fwlink/?LinkID=613722) on how to use this task.
==============================================================================
##[debug]VstsTaskSdk 0.9.0 commit 6c48b16164b9a1c9548776ad2062dad5cd543352
##[debug]Entering D:\a\_tasks\PublishSymbols_0675668a-7bba-4ccb-901d-5ad6554ca653\2.0.14\PublishSymbols.ps1.
##[debug]Loading resource strings from: D:\a\_tasks\PublishSymbols_0675668a-7bba-4ccb-901d-5ad6554ca653\2.0.14\Task.json
##[debug]Loaded 23 strings.
##[debug]SYSTEM_CULTURE: 'en-US'
##[debug]Loading resource strings from: D:\a\_tasks\PublishSymbols_0675668a-7bba-4ccb-901d-5ad6554ca653\2.0.14\Strings\resources.resjson\en-US\resources.resjson
##[debug]Loaded 23 strings.
##[debug]Loading module from path 'D:\a\_tasks\PublishSymbols_0675668a-7bba-4ccb-901d-5ad6554ca653\2.0.14\SymbolsCommon.psm1'.
##[debug]Exporting function 'Get-PdbstrPath'.
##[debug]Exporting function 'Get-DbghelpPath'.
##[debug]Exporting function 'Get-SymStorePath'.
##[debug]Importing function 'Get-DbghelpPath'.
##[debug]Importing function 'Get-PdbstrPath'.
##[debug]Importing function 'Get-SymStorePath'.
##[debug]INPUT_SYMBOLSERVERTYPE: 'TeamServices'
##[debug]INPUT_DETAILEDLOG: 'true'
##[debug] Converted to bool: True
##[debug]INPUT_INDEXSOURCES: 'true'
##[debug] Converted to bool: True
##[debug]INPUT_COMPRESSSYMBOLS: 'false'
##[debug] Converted to bool: False
##[debug]INPUT_TREATNOTINDEXEDASWARNING: 'false'
##[debug] Converted to bool: False
##[debug]BUILD_SOURCESDIRECTORY: 'D:\a\1\s'
##[debug]INPUT_SYMBOLSFOLDER: 'D:\a\1\s'
##[debug]INPUT_SEARCHPATTERN: '___output/**/*.pdb'
##[debug]Entering Find-VstsMatch.
##[debug]DefaultRoot: 'D:\a\1\s'
##[debug]FindOptions.FollowSpecifiedSymbolicLink: 'True'
##[debug]FindOptions.FollowSymbolicLinks: 'True'
##[debug]MatchOptions.Dot: 'True'
##[debug]MatchOptions.FlipNegate: 'False'
##[debug]MatchOptions.MatchBase: 'False'
##[debug]MatchOptions.NoBrace: 'True'
##[debug]MatchOptions.NoCase: 'True'
##[debug]MatchOptions.NoComment: 'False'
##[debug]MatchOptions.NoExt: 'False'
##[debug]MatchOptions.NoGlobStar: 'False'
##[debug]MatchOptions.NoNegate: 'False'
##[debug]MatchOptions.NoNull: 'False'
##[debug]Pattern: '___output/**/*.pdb'
##[debug]FindPath: 'D:\a\1\s\___output'
##[debug]StatOnly: 'False'
##[debug]Path not found.
##[debug]Found 0 paths.
##[debug]Applying include pattern.
##[debug]AdjustedPattern: 'D:\a\1\s\___output/**/*.pdb'
##[debug]Leaving Find-VstsMatch.
##[debug]Leaving D:\a\_tasks\PublishSymbols_0675668a-7bba-4ccb-901d-5ad6554ca653\2.0.14\PublishSymbols.ps1.
##[debug]Caught exception from task script.
##[debug]Error record:
##[debug]Find-VstsMatch : Exception calling "Filter" with "3" argument(s): "Value cannot be null.
##[debug]Parameter name: source"
##[debug]At D:\a\_tasks\PublishSymbols_0675668a-7bba-4ccb-901d-5ad6554ca653\2.0.14\PublishSymbols.ps1:117 char:22
##[debug]+ ... matches = @(Find-VstsMatch -DefaultRoot $SymbolsFolder -Pattern $Sear ...
##[debug]+                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
##[debug]    + CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
##[debug]    + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,Find-VstsMatch
##[debug] 
##[debug]Script stack trace:
##[debug]at Find-Match, D:\a\_tasks\PublishSymbols_0675668a-7bba-4ccb-901d-5ad6554ca653\2.0.14\ps_modules\VstsTaskSdk\FindFunctions.ps1: line 221
##[debug]at <ScriptBlock>, D:\a\_tasks\PublishSymbols_0675668a-7bba-4ccb-901d-5ad6554ca653\2.0.14\PublishSymbols.ps1: line 117
##[debug]at <ScriptBlock>, <No file>: line 1
##[debug]at <ScriptBlock>, <No file>: line 22
##[debug]at <ScriptBlock>, <No file>: line 18
##[debug]at <ScriptBlock>, <No file>: line 1
##[debug]Exception:
##[debug]Microsoft.PowerShell.Commands.WriteErrorException: Exception calling "Filter" with "3" argument(s): "Value cannot be null.
##[debug]Parameter name: source"
##[error]Exception calling "Filter" with "3" argument(s): "Value cannot be null.
Parameter name: source"
##[debug]Processed: ##vso[task.logissue type=error]Exception calling "Filter" with "3" argument(s): "Value cannot be null.%0D%0AParameter name: source"
##[debug]Processed: ##vso[task.complete result=Failed]
##[section]Finishing: publish symbols