diff --git a/solvers/DPStokes/mobility.h b/solvers/DPStokes/mobility.h
index 3384b23..7da91fc 100644
--- a/solvers/DPStokes/mobility.h
+++ b/solvers/DPStokes/mobility.h
@@ -148,14 +148,17 @@ class DPStokes : public libmobility::Mobility {
       this->dppar.zmax += 1.5 * this->dppar.w * h / 2;
     }
     real Lz = this->dppar.zmax - this->dppar.zmin;
+    if (Lz <= 2 * this->dppar.hydrodynamicRadius) {
+      throw std::runtime_error("[DPStokes] The box size in z is too small to "
+                               "fit the particles. Try increasing zmax.");
+    }
     real H = Lz / 2;
     // sets chebyshev node spacing at its coarsest (in the middle) to be h
     real nz_actual = M_PI / (asin(h / H)) + 1;
 
-    // pick nearby N such that 2(Nz-1) has two factors of 2 and is FFT
-    // friendly
-    this->dppar.nz = floor(nz_actual);
-    this->dppar.nz += (int)ceil(nz_actual) % 2;
+    // pick nearby N such that 2(Nz-1) has two factors of 2 and is FFT friendly
+    this->dppar.nz = (int)floor(nz_actual);
+    this->dppar.nz += (this->dppar.nz - 1) % 2;
 
     dpstokes->initialize(dppar);
     Mobility::initialize(ipar);
diff --git a/tests/test_initialization.py b/tests/test_initialization.py
index a001ed6..93a7b12 100644
--- a/tests/test_initialization.py
+++ b/tests/test_initialization.py
@@ -97,3 +97,10 @@ def test_nbody_wall_height_parameter():
         )  # single_wall periodicity requires wall height param
 
     solver.setParameters(wallHeight=0.5)
+
+
+def test_dpstokes_narrow_channel():
+    solver = DPStokes("periodic", "periodic", "two_walls")
+    solver.setParameters(Lx=3.0, Ly=3.0, zmin=0.0, zmax=1.0)
+    with pytest.raises(RuntimeError, match="DPStokes"):
+        solver.initialize(hydrodynamicRadius=1.0, viscosity=1.0)