diff --git a/pyproject.toml b/pyproject.toml
index 3eb8e764..aa46d6a1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -102,6 +102,7 @@ extend-ignore = [
     "module-import-not-at-top-of-file",
     "multiple-spaces-before-operator",
     "non-empty-init-module",
+    "pytest-parameter-with-default-argument",
     "try-consider-else",
     "type-check-without-type-error",
 ]
diff --git a/sumpy/expansion/diff_op.py b/sumpy/expansion/diff_op.py
index 050a7a60..790f9f1a 100644
--- a/sumpy/expansion/diff_op.py
+++ b/sumpy/expansion/diff_op.py
@@ -421,8 +421,7 @@ def as_scalar_pde(
     """
     indices: set[int] = set()
     for eq in pde.eqs:
-        for deriv_ident in eq:
-            indices.add(deriv_ident.vec_idx)
+        indices.update(deriv_ident.vec_idx for deriv_ident in eq)
 
     # this is already a scalar pde
     if len(indices) == 1 and next(iter(indices)) == comp_idx: