diff --git a/src/Solvers/DESolver.cpp b/src/Solvers/DESolver.cpp
index 59e95d9c1f745e4be6787fed74d30b841d56b4b0..83dc750b44d80e52603728952fef83dd159ddd54 100644
--- a/src/Solvers/DESolver.cpp
+++ b/src/Solvers/DESolver.cpp
@@ -16,7 +16,7 @@ MultiIndex::MultiIndex(size_t dimension) {
 }
 
 void MultiIndex::set_partial_derivative(size_t index, size_t value) {
-    this->partial_derivatives_degrees[ index ] = value;
+    this->partial_derivatives_degrees.at( index ) = value;
 }
 
 std::vector<size_t>* MultiIndex::get_partial_derivatives_degrees() {