This is usually a bug in the WCF proxy generation. See here for some details and a workaround.
Taken from comment link:
These are the steps as of January 31, 2012 to fix this issue in Visual Studio for .Net clients:
1) Click the "Show all files" button in the Solution Explorer for the project containing the amazon service reference.
2) Expand the reference and open the AWSECommerceService.wsdl file in the editor
3) On line 584 change the "maxOccurs' to "1".
4) Save the AWSECommerceService.wsdl file
5) Right click Reference.svcmap and click "Run custom tool"
6) Expand Reference.svcmap and open either Reference.cs or Reference.vb
7) Navigate to AmazonAPI.your namespace.Item using the drop down at the top of the window.
8) Navigate to the ImageSets property and confirm that its declaration looks like this:
public ImageSet[] ImageSets {
and NOT like this
public ImageSet[][] ImageSets {
9) Rebuild your project