diff --git a/src/flare_pp/bffs/sparse_gp.cpp b/src/flare_pp/bffs/sparse_gp.cpp
index 698ee6f0..c713f7df 100644
--- a/src/flare_pp/bffs/sparse_gp.cpp
+++ b/src/flare_pp/bffs/sparse_gp.cpp
@@ -191,6 +191,10 @@ void SparseGP ::add_specific_environments(const Structure &structure,
 void SparseGP ::add_uncertain_environments(const Structure &structure,
                                            const std::vector<int> &n_added) {
 
+    if (n_added.size() != n_kernels) {
+        throw std::runtime_error("n_added must have the same size as the number of kernels");
+    }
+
   initialize_sparse_descriptors(structure);
   // Compute cluster uncertainties.
   std::vector<std::vector<int>> sorted_indices =