weird "bytes missing" error for a callback definition

Hello to all,

I am doing a little wrapper for a library that performs some live image acquisition.
This library has a callback mechanism which is called when there is an available live image.

The headers are quite simple:
# ------------------------

DLLIMPORT SetImageCallBack(void (*ptr_ImageCallback)(int, int, char * ) );


This got translated to:
# ------------------------

STRING = c_char_p
SetImageCallBack = _stdcall_libraries['mylib.dll'].SetImageCallBack
SetImageCallBack.restype = c_int
SetImageCallBack.argtypes = [CFUNCTYPE(None, c_int, c_int, STRING)]


However I am getting this error:
# ------------------------

ValueError: Procedure probably called with not enough arguments (4 bytes missing)


This is the simplified code that makes this error happen:
# ------------------------

# initialize the callbacks dictionnary
callbacks = {}

# define python function to be called by the callback
def ViewFinderCallback(int_1, int_2, charp_1):
        print 'viewfindercallback'
        return 0 # return None does the same thing

# declaration
SetImageCallBack = _stdcall_libraries['mylib.dll'].SetImageCallBack
SetImageCallBack.restype = c_int
CallbackFunctionImageCallback = CFUNCTYPE(None, c_int, c_int, STRING)
SetImageCallBack.argtypes = [CallbackFunctionImageCallback]
callbacks['ViewFinderCallback'] = CallbackFunctionImageCallback(self.ViewFinderCallback)

# then registry of the callback
SetImageCallBack(callbacks['ViewFinderCallback'])



This gives me the error ... I dont know what is wrong here. 

I hope someone can help me please,

 		 	   		  
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