PDCollectionSchemaGetField crashes acrobat

I have opened a portfolio and are trying to retrieve information from it by position, the problem is the PDCollectionSchemaGetField routine below is crashing acrobat. I have tried different index positions e.g. 0, 1 etc.  any ideas? I have tried different pdf's also. thanks. lee. 

The (sz) schema length is: 12 

It's says in the documentation:- ASBool PDCollectionSchemaGetField(PDCollectionSchema schema, PDCollectionFieldRec* field) Gets a field by name or position. The caller must set field.size to sizeof(PDCollectionFieldRec). To look up a field by name, set field.fieldName to the appropriate name.  To look up a field by position, set field.fieldName to ASAtomNull, and set field.index to the position. The caller owns (and must destroy) field.fieldText if it is not NULL. 

my code:-       

if(PDCollectionIsValid(pdCollect))     

{           

PDCollectionSchema schema = PDCollectionSchemaAcquire(pdCollect);          

if(schema)          

{              

ASArraySize sz = PDCollectionSchemaGetLength(schema);             

char msg[255] = { '\0' };             

sprintf(msg,"schema size: %ld",sz);             

AVAlertNote(msg);    

PDCollectionFieldRec recSchemaField;             

recSchemaField.size = sizeof(PDCollectionFieldRec);             

recSchemaField.fieldName = ASAtomNull;             

recSchemaField.index = 0;       

AVAlertNote("PDCollectionSchemaGetField");              

ASBool rt = false;        

DURING                        

rt = PDCollectionSchemaGetField(schema, &recSchemaField);   // crashes acrobat here!                   

HANDLER                     

char errorMsg[255] = { '\0' };                               

ASInt32  myException = ASGetExceptionErrorCode ();                               

ASGetErrorString  ( myException, errorMsg, 256);                               

AVAlertNote  ( errorMsg);                              

return;               

END_HANDLER      

AVAlertNote("PDCollectionSchemaGetField ran ok");