diff --git a/sumpy/expansion/__init__.py b/sumpy/expansion/__init__.py
index ea70b0eba96b1fefc2a58a7ac0567fe7c9b5785b..eeaaa650ecf776c276840a83fd8ff080214f82bb 100644
--- a/sumpy/expansion/__init__.py
+++ b/sumpy/expansion/__init__.py
@@ -416,6 +416,8 @@ class LinearPDEBasedExpansionTermsWrangler(ExpansionTermsWrangler):
         pde_dict = self.get_pde().eq
         for ident in pde_dict.keys():
             if ident not in coeff_ident_enumerate_dict:
+                # Order of the expansion is less than the order of the PDE.
+                # In that case, the compression matrix is the identity matrix
                 coeff_matrix = defaultdict(list)
                 reconstruct_matrix = []
                 for i in range(len(mis)):