diff --git a/src/05-poisson-problem.hh b/src/05-poisson-problem.hh
index 8f4cac5662cd6b967d90083250cb5e088b98824b..6c190933ccac8516f547b696db849bd4e4e835c7 100644
--- a/src/05-poisson-problem.hh
+++ b/src/05-poisson-problem.hh
@@ -170,7 +170,7 @@ void assemblePoissonProblemPQ1(
         matrixBuilder[globalI][globalJ] += elementMatrix[i][j];
       }
 
-      rhs[globalI] = elementRhs[i];
+      rhs[globalI] += elementRhs[i];
     }
   }
   matrix.compress();