diff --git a/gamgee/utils/genotype_utils.h b/gamgee/utils/genotype_utils.h
index d209ed1a6..578816d48 100644
--- a/gamgee/utils/genotype_utils.h
+++ b/gamgee/utils/genotype_utils.h
@@ -38,12 +38,15 @@ using namespace std;
   bool allele_missing(const bcf_fmt_t* const format_ptr, const uint8_t* data_ptr, const uint32_t allele_index);
 
   template<class TYPE>
-  inline int32_t allele_key(const uint8_t* data_ptr, const uint32_t allele_index, const TYPE missing) {
+  inline int32_t allele_key(const uint8_t* data_ptr, const uint32_t allele_index, const TYPE missing, const TYPE vector_end) {
     // mostly copied from htslib
     const auto p = reinterpret_cast<const TYPE*>(data_ptr);
     if ( !(p[allele_index]>>1) || p[allele_index]==missing ) {
       return missing_values::int32;
     }
+    else if ( p[allele_index] == vector_end ) {
+      return bcf_int32_vector_end;
+    }
     return (p[allele_index]>>1)-1;
   }
 
@@ -59,11 +62,11 @@ using namespace std;
       const bcf_fmt_t* const format_ptr, const uint8_t* data_ptr, const uint32_t allele_index) {
     switch (format_ptr->type) {
     case BCF_BT_INT8:
-      return allele_key<int8_t>(data_ptr, allele_index, bcf_int8_missing);
+      return allele_key<int8_t>(data_ptr, allele_index, bcf_int8_missing, bcf_int8_vector_end);
     case BCF_BT_INT16:
-      return allele_key<int16_t>(data_ptr, allele_index, bcf_int16_missing);
+      return allele_key<int16_t>(data_ptr, allele_index, bcf_int16_missing, bcf_int16_vector_end);
     case BCF_BT_INT32:
-      return allele_key<int32_t>(data_ptr, allele_index, bcf_int32_missing);
+      return allele_key<int32_t>(data_ptr, allele_index, bcf_int32_missing, bcf_int32_vector_end);
     default:
       throw invalid_argument("unknown GT field type: " + to_string(format_ptr->type));
     }
diff --git a/test/genotypes_test.cpp b/test/genotypes_test.cpp
index ce4e533ae..fdfb0d4cc 100644
--- a/test/genotypes_test.cpp
+++ b/test/genotypes_test.cpp
@@ -154,3 +154,15 @@ BOOST_AUTO_TEST_CASE( is_variant ) {
     BOOST_CHECK_EQUAL(genotypes[i].variant(), truth[i]);
 }
 
+BOOST_AUTO_TEST_CASE( mixed_ploidy_random_access ) {
+  const auto rec = (*(SingleVariantReader("testdata/test_variants_mixed_ploidy.vcf").begin()));
+  const auto expected = vector<vector<int32_t>>{ {0, 1}, {0, bcf_int32_vector_end}, {1, bcf_int32_vector_end} };
+
+  auto sample_idx = 0u;
+  for ( const auto& genotype : rec.genotypes() ) {
+    for ( auto allele_idx = 0u; allele_idx < genotype.size(); ++allele_idx ) {
+      BOOST_CHECK_EQUAL(genotype[allele_idx], expected[sample_idx][allele_idx]);
+    }
+    ++sample_idx;
+  }
+}
diff --git a/testdata/test_variants_mixed_ploidy.vcf b/testdata/test_variants_mixed_ploidy.vcf
new file mode 100644
index 000000000..54229ac73
--- /dev/null
+++ b/testdata/test_variants_mixed_ploidy.vcf
@@ -0,0 +1,19 @@
+##fileformat=VCFv4.1
+##INFO=<ID=AF,Number=A,Type=Float,Description="Allele Frequency, for each ALT allele, in the same order as listed">
+##INFO=<ID=AN,Number=1,Type=Integer,Description="Total number of alleles in called genotypes">
+##INFO=<ID=VALIDATED,Number=0,Type=Flag,Description="Validated By Follow-up Experiment">
+##INFO=<ID=DESC,Number=.,Type=String,Description="Custom Description">
+##FILTER=<ID=PASS,Description=All filters passed>
+##FILTER=<ID=LOW_QUAL,Description=Low quality call>
+##FILTER=<ID=MISSED,Description=Missed by the variant caller>
+##FILTER=<ID=NOT_DEFINED,Description=Undefined filter>
+##contig=<ID=1,Length=300000000,Description=the first chromosome>
+##contig=<ID=20,Length=64000000>
+##contig=<ID=22,Length=120000000>
+##FORMAT=<ID=GT,Number=1,Type=String,Description=Genotype>
+##FORMAT=<ID=GQ,Number=1,Type=Integer,Description=Genotype quality>
+##FORMAT=<ID=PL,Number=G,Type=Integer,Description=Phred scaled relative Likelihoods of the genotypes>
+##FORMAT=<ID=AF,Number=2,Type=Float,Description=Arbitrary float field with 2 values (test purposes only)>
+##FORMAT=<ID=AS,Number=1,Type=String,Description=Arbitrary string field with 1 value (test purposes only)>
+#CHROM	POS	ID	REF	ALT	QUAL	FILTER	INFO	FORMAT	NA12878	NA12891	NA12892
+1	10000000	db2342	T	C	80	PASS	AF=0.5;AN=6;VALIDATED;DESC=Test1,Test2	GT:GQ:PL:AF:AS	0/1:25:10,0,100:3.1,2.2:ABA	0:12:0,10,1000:3.1,2.2:CA	1:650:10,100,0:3.1,2.2:XISPAFORRO