Windows Service - The module was expected to contain an assembly manifest

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Guys - Having trouble deploying a windows service via the installutil command line. It returns the following error:

Exception occurred while initializing the installation:

System.BadImageFormatException: Could not load file or assembly xxxxx or one of its dependencies. The module was expected to contain an assembly manifest

Target environment is x64, and deploying on Windows Server 2008 DataCenter x64.