Marshaling a multi-dimensional array

[Tom Kent]

I have a dll that is expecting a 3x3 array which I'm am using DllImport to access. In addition, this array is located in a struct that is getting passed. I'm having problems that I think are located somewhere in how I'm marshaling it. Any input would be appreciated.
(examples are shortened for clarity, let me know if something more complete would be useful)

/*C code*/
typedef float RotMtxType[3][3];
typedef struct {
double ccNum;
RotMtxType ccuGCCtoLNEDrotMtx;
}

extern int ccuInitStateStructure(
CCUStateStruct *ccuStatePtr);

//C# Code
[StructLayout(LayoutKind.Sequential)]
public struct CCUStateStruct
{

public double ccNum;
[MarshalAs(UnmanagedType.ByValArray, SizeConst=9]
public float [,] ccuGCCtoLNEDDrotMtx;
}

elsewhere ...
ccuState.ccuGCCtoLNEDDrotMtx = new float[3,3]
....

[DllImport("CCU.dll")]
public extern static int ccuInitStateStructure (
[MarshalAs(UnmanagedType.Struct)]ref CCUStateStruct ccuStatePtr);
I then get the error:
Additional information: Can not marshal field ccuGCCtoLNEDrotMtx of type CCUStateStruct: This type can not be marshaled as a structure field.
Thanks much!
Tom