Using AVAssetWriter to re-encode H264 mov file - how to set frame-rate?

I'm attempting to re-encode an input MOV file with changeable frame-rate and clipped in duration, in iOS. At the moment I have an AVAssetWriter setting video properties a bit like this:

NSMutableDictionary* compressionPropertiesDict = [NSMutableDictionary new];
compressionPropertiesDict[AVVideoProfileLevelKey] = AVVideoProfileLevelH264High40;

if(self.fps > 0) {
    compressionPropertiesDict[AVVideoAverageNonDroppableFrameRateKey] = [NSNumber numberWithInt:self.fps];


_sessionMgr.videoSettings = @
{
AVVideoCodecKey: AVVideoCodecH264,
AVVideoWidthKey: [NSNumber numberWithFloat:self.outputSize.width],
AVVideoHeightKey: [NSNumber numberWithFloat:self.outputSize.height],
AVVideoCompressionPropertiesKey: compressionPropertiesDict,
};

That comes out at runtime looking like this:

videoSettings = 
{
AVVideoCodecKey = avc1;
AVVideoCompressionPropertiesKey =     {
    AverageNonDroppableFrameRate = 15;
    ProfileLevel = "H264_High_4_0";
};
AVVideoHeightKey = 960;
AVVideoWidthKey = 640;
}

At the end of which, I get a crash with NSInvalidArgumentException: "Compression property AverageNonDroppableFrameRate is not supported for video codec type avc1". (In unit tests using the simulator.)

There's only one codec type that's useable in iOS, AVVideoCodecH264 / "avc1" - and I notice other projects have used the AVVideoAverageNonDroppableFrameRateKey. In fact, I'm using SDAVAssetExportSession and in that codebase I see explicit use of this key. So I would have thought there must be a way to use this key to set frame-rate..?

I've also experimented a bit using AVVideoMaxKeyFrameIntervalKey instead, but that doesn't change my frame-rate at all...

So, to summarise, can anyone help me with setting a different (always lower) output frame-rate for an iOS AVFoundation-based video conversion? Thanks!