diff --git a/sumpy/expansion/__init__.py b/sumpy/expansion/__init__.py
index 3ac64e9e166d82abf60cbd8c9537a1742f569fa9..0523396073cdab505ae47f578ded2b4e50ba1743 100644
--- a/sumpy/expansion/__init__.py
+++ b/sumpy/expansion/__init__.py
@@ -600,6 +600,8 @@ def process_pde(pde):
                     multiplier = m
                 elif multiplier != m:
                     return pde, 1
+    if multiplier is None:
+        return pde, 1
     eqs = []
     for eq in pde.eqs:
         new_eq = dict()