diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index f3ba9f4..ed7a984 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -91,7 +91,7 @@ jobs:
         fi
         popd
         pushd src/testdata
-        conda install --yes -c conda-forge h5py
+        conda install --yes -c conda-forge h5py python=3.10
         # weighted_unnormalized
         time ssu -m weighted_unnormalized_fp32 -i test500.biom  -t test500.tre --pcoa 4  -r hdf5_fp32 -o t1.h5
         ./compare_unifrac_matrix.py test500.weighted_unnormalized_fp32.h5 t1.h5 1.e-5
diff --git a/src/api.cpp b/src/api.cpp
index 83af3a6..7e69a13 100644
--- a/src/api.cpp
+++ b/src/api.cpp
@@ -510,14 +510,33 @@ compute_status faith_pd_one_off(const char* biom_filename, const char* tree_file
     SETUP_TDBG("faith_pd_one_off")
     CHECK_FILE(biom_filename, table_missing)
     CHECK_FILE(tree_filename, tree_missing)
-    PARSE_SYNC_TREE_TABLE(tree_filename, table_filename)
-
+    PARSE_TREE_TABLE(tree_filename, biom_filename)
     TDBG_STEP("load_files")
-    initialize_results_vec(*result, table);
 
-    // compute faithpd
-    su::faith_pd(table, tree_sheared, std::ref((*result)->values));
-    TDBG_STEP("faith_pd")
+    // Filter out any elements with zero counts
+    su::biom_inmem table_nz(table,1.0);
+    if ((table_nz.n_samples==0) || (table_nz.n_obs==0)) {
+      fprintf(stderr, "WARNING: All samples had zero counts. Forcing zero result.\n");
+      SYNC_TREE_TABLE(tree, table)
+
+      TDBG_STEP("sync_tree_table")
+      initialize_results_vec(*result, table);
+      // nothing else to do... results already initialized to 0
+      TDBG_STEP("faith_pd")
+    } else {
+      if ((table_nz.n_samples!=table.n_samples) || (table_nz.n_obs!=table.n_obs)) {
+        fprintf(stderr, "WARNING: Some samples had zero counts and were filtered out.\n");
+      }
+      SYNC_TREE_TABLE(tree, table_nz)
+
+      TDBG_STEP("sync_tree_table")
+    
+      initialize_results_vec(*result, table_nz);
+
+      // compute faithpd
+      su::faith_pd(table_nz, tree_sheared, std::ref((*result)->values));
+      TDBG_STEP("faith_pd")
+    }
 
     return okay;
 }
diff --git a/src/biom_inmem.cpp b/src/biom_inmem.cpp
index b444dea..69a8d61 100644
--- a/src/biom_inmem.cpp
+++ b/src/biom_inmem.cpp
@@ -257,8 +257,9 @@ biom_inmem::biom_inmem(const biom_inmem &other, const double min_sample_counts)
   , sample_ids()
   , obs_ids()
 {
-    #pragma omp parallel for schedule(static)
-    for(int i = 0; i < 2; i++) {
+    if ((resident_obj.n_obs>0) && (resident_obj.n_samples>0)) {
+     #pragma omp parallel for schedule(static)
+     for(int i = 0; i < 2; i++) {
         if (i==0) {
           // filter out zero obs
           uint32_t obs_cnt = 0;
@@ -293,7 +294,14 @@ biom_inmem::biom_inmem(const biom_inmem &other, const double min_sample_counts)
            }
            create_id_index(sample_ids, sample_id_index);
         }
-    }
+     }
+   } //((resident_obj.n_obs>0) && (resident_obj.n_samples>0))
+   else
+   {
+     // degenerate case, just set to 0
+     n_samples = 0;
+     n_obs = 0;
+   }
 }
 
 
diff --git a/src/unifrac.cpp b/src/unifrac.cpp
index 6d3bc4b..e2ea020 100644
--- a/src/unifrac.cpp
+++ b/src/unifrac.cpp
@@ -369,7 +369,8 @@ void su::faith_pd(biom_interface &table,
     double length;
 
     // for node in postorderselect
-    for(unsigned int k = 0; k < (tree.nparens / 2) - 1; k++) {
+    const unsigned int max_k = (tree.nparens>1) ? ((tree.nparens / 2) - 1) : 0;
+    for(unsigned int k = 0; k < max_k; k++) {
         node = tree.postorderselect(k);
         // get branch length
         length = tree.lengths[node];
diff --git a/src/unifrac_cmp.cpp b/src/unifrac_cmp.cpp
index e40c9e8..7ae65e6 100644
--- a/src/unifrac_cmp.cpp
+++ b/src/unifrac_cmp.cpp
@@ -137,7 +137,7 @@ inline void unifracTT(const su::biom_interface &table,
          */
 
     unsigned int k = 0; // index in tree
-    const unsigned int max_k = (tree.nparens / 2) - 1;
+    const unsigned int max_k = (tree.nparens>1) ? ((tree.nparens / 2) - 1) : 0;
 
     const unsigned int num_prop_chunks = propstack_multi.get_num_stacks();
     while (k<max_k) {
@@ -277,7 +277,7 @@ inline void unifrac_vawTT(const su::biom_interface &table,
     TFloat *lengths = taskObj.lengths;
 
     unsigned int k = 0; // index in tree
-    const unsigned int max_k = (tree.nparens / 2) - 1;
+    const unsigned int max_k = (tree.nparens>1) ? ((tree.nparens / 2) - 1) : 0;
 
     const unsigned int num_prop_chunks = propstack_multi.get_num_stacks();
     while (k<max_k) {