diff --git a/sumpy/expansion/__init__.py b/sumpy/expansion/__init__.py
index 704490746842b46ebbbd414e94fe6b39fe33bcc4..b335917c3eb87b94d3eadc7bb2c30de8a82881a0 100644
--- a/sumpy/expansion/__init__.py
+++ b/sumpy/expansion/__init__.py
@@ -490,7 +490,7 @@ class HelmholtzDerivativeWrangler(LinearRecurrenceBasedDerivativeWrangler):
             mi = [0]*self.dim
             mi[d] = 2
             pde_dict[tuple(mi)] = 1
-        pde_dict[tuple([0]*self.dim)] = -1
+        pde_dict[tuple([0]*self.dim)] = 1
         return pde_dict
 
     def get_reduced_coeffs(self):