diff --git a/meshmode/mesh/processing.py b/meshmode/mesh/processing.py
index 0c0a8e8db429224488ec483e322821dcaa4470ab..cfdec7fa4a32255def70473e7175186bcebbaff2 100644
--- a/meshmode/mesh/processing.py
+++ b/meshmode/mesh/processing.py
@@ -273,9 +273,15 @@ def merge_dijsoint_meshes(meshes, skip_tests=False):
 
 # {{{ affine map
 
-def affine_map(mesh, A=None, b=None):
+def affine_map(mesh, A=None, b=None):  # noqa
     """Apply the affine map *f(x)=Ax+b* to the geometry of *mesh*."""
 
+    if A is None:
+        A = np.eye(mesh.ambient_dim)  # noqa
+
+    if b is None:
+        b = np.zeros(A.shape[0])
+
     vertices = np.einsum("ij,jv->iv", A, mesh.vertices) + b[:, np.newaxis]
 
     # {{{ assemble new groups list