The MAC signature found in the HTTP request '...' is not the same as any computed signature

I'm sending the following request in Postman to retrieve a simple .jpg from Azure Blob storage at this URL https://steamo.blob.core.windows.net/testcontainer/dog.jpg

GET /testcontainer/dog.jpg HTTP/1.1Host: steamo.blob.core.windows.netAuthorization: SharedKey steamo:<my access key>x-ms-date: Tue, 26 May 2015 17:35:00 GMTx-ms-version: 2014-02-14Cache-Control: no-cachePostman-Token: b1134f8a-1a03-152c-2810-9cb351efb9ce

If you're unfamiliar with Postman it is just a REST client - the Postman-Token header can probably be ignored.

My access key is copied from my Azure Management Portal.

I get this error:

Server failed to authenticate the request. Make sure the value of Authorization header is formed correctly including the signature. RequestId:2482503d-0001-0033-60da-9708ed000000 Time:2015-05-26T17:35:41.4577821Z

With this AutheticationErrorDetail:

The MAC signature found in the HTTP request '<my access key>' is not the same as any computed signature. Server used following string to sign: 'GET x-ms-date:Tue, 26 May 2015 17:35:00 GMT x-ms-version:2014-02-14 /steamo/testcontainer/dog.jpg'.

How do I fix this? Let me know if you need any more info from me.