Unable to use add or update AllProperties to add/update property bags: getting Error already added but actually never added.

Hi All,

Unable to use add or update AllProperties to add/update property bags: getting Error already added but actually never added.

Able to add properties and property bags only using

$web.Properties["smallcasekeyonly"] = "value";
$web.Properties.Update();

But, Unable to use add or update AllProperties to add/update property bags: getting Error already added but actually never added.
$web.AllProperties["CaseSensitiveKey"] = "Value";

$web.Update();

Please help me on this.

Regards!

Keshav