unmanaged structure layout error

I'm having a problem with an unmanaged structure.

I'm getting this error message when the system tries to load the type:

*Could not load type '_S' from assembly '...' because it contains an object
field at offset 134 that is incorrectly aligned or overlapped by a
non-object field.*

For the life of me I cannot figure this out why this is failing.
I've set the byte packing to 1 which means that there is no padding between
bytes.  I've also explicitly specified each field offset.

Below is the structure that I have declared.  It's only a partial structure
and I removed all the properties for brevity.  Also there's more members but
the last field is where the error is occurring.  Also, if I remove the last
field it works.  When I add it back it fails.

Any suggestions?

Thanks,
Mike

[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi, Pack = 1)]
private struct _S {
    [FieldOffset(0)]
    private System.Double _height;
    [FieldOffset(8)]
    private System.Double _width;
    [FieldOffset(16)]
    private System.Double _depth;
    [FieldOffset(24)]
    private System.Double _a;
    [FieldOffset(32)]
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 24)]
    private System.Byte[] _u1;
    [FieldOffset(56)]
    private System.UInt16 _number;
    [FieldOffset(58)]
    private System.UInt16 _d;
    [FieldOffset(60)]
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 4)]
    private System.Byte[] _u2;
    [FieldOffset(64)]
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 70)]
    private System.String _name;
    [FieldOffset(134)]
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 33)]
    private System.String _address1;

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