diff --git a/faiss/gpu/GpuIcmEncoder.cu b/faiss/gpu/GpuIcmEncoder.cu
index 434fae9e36..8bd60f91b8 100644
--- a/faiss/gpu/GpuIcmEncoder.cu
+++ b/faiss/gpu/GpuIcmEncoder.cu
@@ -82,7 +82,7 @@ void GpuIcmEncoder::encode(
         size_t n,
         size_t ils_iters) const {
     size_t nshards = shards->size();
-    size_t shard_size = (n + nshards - 1) / nshards;
+    size_t base_shard_size = n / nshards;
 
     auto codebooks = lsq->codebooks.data();
     auto M = lsq->M;
@@ -94,8 +94,14 @@ void GpuIcmEncoder::encode(
 
     // split input data
     auto fn = [=](int idx, IcmEncoderImpl* encoder) {
-        size_t i0 = idx * shard_size;
-        size_t ni = std::min(shard_size, n - i0);
+        size_t i0 = idx * base_shard_size + std::min(size_t(idx), n % nshards);
+        size_t ni = base_shard_size;
+        if (ni < n % nshards) {
+            ++ni;
+        }
+        if (ni <= 0) { // only if n < nshards
+            return;
+        }
         auto xi = x + i0 * d;
         auto ci = codes + i0 * M;
         std::mt19937 geni(idx + seed); // different seed for each shard