You cannot call a method on a null-valued expression. (Invoke-PasswordSprayOWA)

when i do this command : ( Invoke-PasswordSprayOWA -ExchHostname mail.**.com -Domain *** -UserList .\user
s.txt -Password ** )

return :

[] Now spraying the OWA portal at https://mail.****.com/owa/
[] Current date and time: 10/24/2017 23:36:44
You cannot call a method on a null-valued expression.
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull
+ PSComputerName : localhost

You cannot call a method on a null-valued expression.
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull
+ PSComputerName : localhost

You cannot call a method on a null-valued expression.
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull
+ PSComputerName : localhost

You cannot call a method on a null-valued expression.
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull
+ PSComputerName : localhost

You cannot call a method on a null-valued expression.
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull
+ PSComputerName : localhost

what should i do to resolve this issue.

Start by reproducing Invoke-PasswordSprayOWA with the reported Exchange hostname, domain, and user list parameters. Trace the function's request and response handling to identify which value is null when the method is called. Done means the command handles this response without InvokeMethodOnNull errors and reports the authentication result appropriately.