Invalid Soft Mask - help required

I am trying to add an alpha channel to an in-line image using a soft mask in the graphics state. When the soft mask is referenced directly from an XObject image it works fine, but when referenced from the ExtGState object it throws the above error when the document is opened in reader. A skeleton of the code is below, and any help is appreciated.

9 0 obj
<</Type /XObject
/Subtype /Image
/Width 14
/Height 12
/BitsPerComponent 8
/Name /I9
/Length 71
/ColorSpace /DeviceGray
/Filter /FlateDecode
>>
stream
..................
endstream
endobj

10 0 obj
<<
/Type /ExtGState
/SMask 9 0 R
>>
endobj

47 0 obj
<< /Type /Page
/Parent 89 0 R
/MediaBox [0 0 1190.55 841.89]
/Resources
<<
/Font << /Helvetica 4 0 R /Helvetica-Bold 3 0 R >>
/ExtGState << /GS10 10 0 R >>
/ProcSet [/PDF/ImageC/ImageI/ImageB/Text]
/XObject << ........... >>
>>
/Contents [5 0 R 6 0 R 46 0 R ]
>>
endobj

...............

q
/GS10 gs
14 0 0 9 262.75 722.39 cm
BI
/Width 14
/Height 12
/BitsPerComponent 8
/Name /I8
/ColorSpace /DeviceRGB
/Filter [/A85 /FlateDecode]
/Length 13
ID
Gar7\"X=W[!!2ip!<~>
EI
Q