diff --git a/boxtree/pyfmmlib_integration.py b/boxtree/pyfmmlib_integration.py
index da56c1e38bdc3baa34675d7039f3f215f7bff3be..47c617d704426030bbc02eb7e47542672b024d73 100644
--- a/boxtree/pyfmmlib_integration.py
+++ b/boxtree/pyfmmlib_integration.py
@@ -682,6 +682,11 @@ class FMMLibExpansionWrangler(object):
 
         m2l_rotation_angles = self.rotation_data.m2l_rotation_angles()
 
+        if len(m2l_rotation_angles) == 0:
+            # The pyfmmlib wrapper may or may not complain if you give it a
+            # zero-length array.
+            return (rotmatf, rotmatb, rotmat_order)
+
         def mem_estimate(order):
             # Rotation matrix memory cost estimate.
             return (8
diff --git a/test/test_fmm.py b/test/test_fmm.py
index 2bd3f21c46c3959c16d5a804af41f01b54f39171..b5d2e1a5964e6fb15d7909f42d7496d6054fb56c 100644
--- a/test/test_fmm.py
+++ b/test/test_fmm.py
@@ -725,7 +725,9 @@ def test_fmm_float32(ctx_factory, enable_extents):
 
 @pytest.mark.parametrize("well_sep_is_n_away", (1, 2))
 @pytest.mark.parametrize("helmholtz_k", (0, 2))
-def test_fmm_with_optimized_3d_m2l(ctx_factory, helmholtz_k, well_sep_is_n_away):
+@pytest.mark.parametrize("nsrcntgts", (20, 10000))
+def test_fmm_with_optimized_3d_m2l(ctx_factory, nsrcntgts, helmholtz_k,
+                                   well_sep_is_n_away):
     logging.basicConfig(level=logging.INFO)
 
     from pytest import importorskip
@@ -736,8 +738,7 @@ def test_fmm_with_optimized_3d_m2l(ctx_factory, helmholtz_k, well_sep_is_n_away)
     ctx = ctx_factory()
     queue = cl.CommandQueue(ctx)
 
-    nsources = 5000
-    ntargets = 5000
+    nsources = ntargets = nsrcntgts // 2
     dtype = np.float64
 
     sources = p_normal(queue, nsources, dims, dtype, seed=15)