diff --git a/src/ConcreteOperations/exact_sum.jl b/src/ConcreteOperations/exact_sum.jl
index 2625178e30..50146f90f4 100644
--- a/src/ConcreteOperations/exact_sum.jl
+++ b/src/ConcreteOperations/exact_sum.jl
@@ -13,11 +13,20 @@ Compute the exact sum of sparse polyomial zonotopes ``P₁`` and ``P₂``.
 ### Output
 
 A `SparsePolynomialZonotope` representing the exact sum ``P₁ ⊞ P₂``.
+
+### Algorithm
+
+This method implements Proposition 3.1.20 in [1].
+
+[1] Kochdumper, Niklas. *Extensions of polynomial zonotopes and their application
+    to verification of cyber-physical systems.* PhD diss., Technische Universität
+    München, 2022.
 """
 function exact_sum(P1::SparsePolynomialZonotope, P2::SparsePolynomialZonotope)
-    indexvector(P1) == indexvector(P2) || throw(ArgumentError("the exact sum " *
-                                                              "is currently only implemented for sparse polynomial zonotopes with " *
-                                                              "the same index vector"))
+    if indexvector(P1) != indexvector(P2)
+        throw(ArgumentError("the exact sum is currently only implemented for " *
+                            "sparse polynomial zonotopes with the same index vector"))
+    end
 
     c = center(P1) + center(P2)
     G = hcat(genmat_dep(P1), genmat_dep(P2))