How to fix "Assertion failed" when training cv:ml::DTrees - c++

Question

I want to train a decision tree with hog features.
When I try to train my cv::ml::DTrees model, I receive the following error:
OpenCV(3.4.3) Error: Assertion failed (n >= 0) in cv::ml::TrainDataImpl::getValues, file C:\build\3_4_winpack-build-win64-vc14\opencv\modules\ml\src\data.cpp, line 890
I understand, that the number of samples(features) have to match the number of responses(labels) and both have to be >=0.
My cv::Mat feats and cv::Mat labels have the same number of rows (388).
This is how I try to do it:
cv::Ptr<cv::ml::DTrees> model = cv::ml::DTrees::create();
cv::Ptr<cv::ml::TrainData> data = cv::ml::TrainData::create(feats, cv::ml::ROW_SAMPLE, labels);
model->train(data);
model->train(data) fails, when it calls this function:
void getValues( int vi, InputArray _sidx, float* values ) const CV_OVERRIDE
{
Mat sidx = _sidx.getMat();
int i, n = sidx.checkVector(1, CV_32S), nsamples = getNSamples();
CV_Assert( 0 <= vi && vi < getNAllVars() );
CV_Assert( n >= 0 );
const int* s = n > 0 ? sidx.ptr<int>() : 0;
if( n == 0 )
n = nsamples;
}
}
Can anyone point me in the right direction to fix this problem?
EDIT:
If I set the MinSampleCount to 388 (the number of samples) I don't get an error, but the prediction won't work correctly (it always returns label 5).