fake va_list in ARC

I need to create in an iOS application a fake va_list to pass to a NSString initWithFormat:arguments: function, this is my code:

NSArray *fixedArguments = [[NSArray alloc] initWithArray:arguments]; 

NSRange range = NSMakeRange(0, [fixedArguments count]);

va_list fakeArgList = (va_list)malloc(sizeof(NSString *) * [fixedArguments count]);

__unsafe_unretained id *ptr = (__unsafe_unretained id *)fakeArgList;

[fixedArguments getObjects:ptr range:range];

content = [[NSString alloc] initWithFormat:outputFormat
                                            arguments:(va_list)fakeArgList];
free(fakeArgList);

The compiler complains with this message on the cast line:

error: cast of a non-Objective-C pointer type 'va_list' (aka 'char *') to '__unsafe_unretained id *' is disallowed with ARC

The getObjects:range: function is defined as follows:

- (void)getObjects:(id __unsafe_unretained [])objects range:(NSRange)range;

I've tried everything but still can't get rid of this error...

Is there a solution for creating a fake va_list with ARC enabled? What am i doing wrong?