FIX: fixed issue #2

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() {