Error regarding "Internally simulated 0 samples from 0 randomly selected reference parents." on GRCh38 build

[QingData]

Hi.
I recently try to use Rfmix2 on a set of 887 sample on a gene region on chr6 (phased vcf file only covers a region on chr6, (8492 variants). As the data is on hg38, I am using the phased vcf from 1kg 30x dataset, more specifically, only the vcf for chr6. Here is the reference for the reference data (https://www.internationalgenome.org/data-portal/data-collection/30x-grch38). My code ran into an error at the function "random-forest.cpp:248:" node_t* add_node(): Assertion `n_ref != 0' failed.".

When I checked the Rfmix printout log, I noticed a line says "Internally simulated 0 samples from 0 randomly selected reference parents." This seems odd, as jobs from my other dataset usually says "Internally simulated 400 samples from 21 randomly selected reference parents..."

Does this error message mean that the problem lies in the reference file? or the sample vcf files? Does rfmix require more variants in the sample vcf file? Maybe those variants (8492 of them) are not covered in the reference file?

FYI, the reference files contain only the founders and are phased.
Thanks a lot.

Below is my code to run Rfmix

'rfmix
-f $lab_phased_vcf
-r $ref_1kg_III_highCov
-m $ref_smap
-g $ref_gmap_hg38_rfmix
-o ${out_path}/$f_out_prefix
--chromosome=chr6
--n-threads=20 '

Below are the error message

**Loading genetic map for chromosome chr6 ... done
Mapping samples ... 887 samples combined
Scanning input VCFs for common SNPs on chromosome chr6 ... 8492 SNPs
Loading haplotypes... done
Defining and initializing conditional random field...
setting up CRF points and random forest windows...
computing random forest window spacing overlay...
initializing apriori reference subpop across CRF...
setting up random forest probability estimation arrays... done
Defining and initializing conditional random field... done
1325854 (8.8%!)(MISSING) variant alleles 0 (0.0%!)(MISSING) missing alleles

Generating internal simulation samples...
Internally simulated 0 samples from 0 randomly selected reference parents.
mm::allocate() called at setup_ref_haplotypes():627 (random-forest.cpp) requesting zero or negative size (0)
mm::allocate() called at setup_ref_haplotypes():683 (random-forest.cpp) requesting zero or negative size (0)
mm::allocate() called at bootstrap_haplotypes():462 (random-forest.cpp) requesting zero or negative size (0)
mm::allocate() called at bootstrap_haplotypes():463 (random-forest.cpp) requesting zero or negative size (0)
mm::allocate() called at build_tree():495 (random-forest.cpp) requesting zero or negative size (0)
rfmix: random-forest.cpp:248: node_t* add_node(tree_t*, int*, int, int*, int, double, mm*, int): Assertion `n_ref != 0' failed.**