diff --git a/src/tests/test_nn_search/main.cpp b/src/tests/test_nn_search/main.cpp
index 3c2ecf473cb..f6efd6ce7cd 100644
--- a/src/tests/test_nn_search/main.cpp
+++ b/src/tests/test_nn_search/main.cpp
@@ -131,7 +131,9 @@ int main(int argc, char** argv) {
         for(index_t i = 0; i < M.vertices.nb(); ++i) {
             const double* q = M.vertices.point_ptr(i);
 
+            neigh1.assign(nb_neigh,NO_INDEX);
             sq_dist1.assign(nb_neigh,0.0);
+            neigh2.assign(nb_neigh,NO_INDEX);
             sq_dist2.assign(nb_neigh,0.0);
             
             if(by_index) {
@@ -149,6 +151,12 @@ int main(int argc, char** argv) {
                     nb_neigh, q, neigh2.data(), sq_dist2.data()
                 );
             }
+
+            for(index_t j=0; j < nb_neigh; ++j) {
+                //std::cerr << i << " " << j << "    " << neigh1[j] << " " << neigh2[j] << std::endl;
+                geo_assert(neigh1[j] != NO_INDEX);
+                geo_assert(neigh2[j] != NO_INDEX);
+            }
             
             bool has_mismatch = false;
 
@@ -158,21 +166,19 @@ int main(int argc, char** argv) {
             //
             // --> Did not work, NOT UNDERSTOOD YET (probably need to play with compilation flags)
             // --> Seems that indices neigh2[] are random
-            /*
             for(index_t j=0; j < nb_neigh; ++j) {
                 index_t nn = neigh2[j];
                 sq_dist2[j] = annDist(
                     M.vertices.dimension(), M.vertices.point_ptr(i), M.vertices.point_ptr(nn)
                 );
             }
-            */
             
             for(index_t j=0; j < nb_neigh; ++j) {
                 // Added tolerance: on Mac/M1 we got tiny differences,
                 // I think it is doing auto FMA here and there, to be
                 // checked.
-                if(::fabs(sq_dist1[j] - sq_dist2[j]) > 1e-6) {
-                // if(sq_dist1[j] != sq_dist2[j]) {
+                // if(::fabs(sq_dist1[j] - sq_dist2[j]) > 1e-6) {
+                if(sq_dist1[j] != sq_dist2[j]) {
                     has_mismatch = true;
                     match = false;
                     Logger::err("Mismatch") << i << "[" << j << "]"
diff --git a/src/tests/test_nn_search/nn_search_ANN.cpp b/src/tests/test_nn_search/nn_search_ANN.cpp
index 936857dda9a..416c8a1c724 100644
--- a/src/tests/test_nn_search/nn_search_ANN.cpp
+++ b/src/tests/test_nn_search/nn_search_ANN.cpp
@@ -94,11 +94,18 @@ namespace GEO {
         index_t* neighbors,
         double* neighbors_sq_dist
     ) const {
+        // In Gargantua mode, index_t is 64 bits, and ANNidx is always 32 bits, so
+        // we need to allocate space for ANN indices, then convert and copy them
+        // to client's neighbors array. 
+        ANNidxArray ann_neighbors = ANNidxArray(alloca(sizeof(ANNidx)*nb_neighbors));
         ann_tree_->annkSearch(
             const_cast<double*>(query_point),
-            int(nb_neighbors), (ANNidxArray) neighbors, neighbors_sq_dist,
+            int(nb_neighbors), ann_neighbors, neighbors_sq_dist,
             (exact_ ? 0.0 : 0.1)
         );
+        for(index_t i=0; i<nb_neighbors; ++i) {
+            neighbors[i] = index_t(ann_neighbors[i]);
+        }
     }
 
     NearestNeighborSearch_ANN::~NearestNeighborSearch_ANN() {