diff --git a/numpy_rms/fallback.py b/numpy_rms/fallback.py
index c634128..c5e4177 100644
--- a/numpy_rms/fallback.py
+++ b/numpy_rms/fallback.py
@@ -19,6 +19,6 @@ def rms_numpy(a: NDArray, window_size: int) -> NDArray:
     output_i = 0
     for offset in range(0, end_index, window_size):
         rms = calculate_rms(a[..., offset : offset + window_size])
-        output_array[output_i] = rms
+        output_array[..., output_i] = rms
         output_i += 1
     return output_array
diff --git a/tests/test_rms.py b/tests/test_rms.py
index ee04c9e..67e75fd 100644
--- a/tests/test_rms.py
+++ b/tests/test_rms.py
@@ -14,6 +14,16 @@ def test_rms_large_array(benchmark):
     assert rms[1] == pytest.approx(7637.1353)
 
 
+def test_rms_2d_mono():
+    arr = np.ones(shape=(1, 1000), dtype=np.float32)
+    rms = numpy_rms.rms(arr, window_size=50)
+    rms_numpy_fallback = rms_numpy(arr, window_size=50)
+    assert rms.shape == (1, 20)
+    assert rms_numpy_fallback.shape == (1, 20)
+    assert rms[0, 0] == 1.0
+    assert_array_almost_equal(rms, rms_numpy_fallback)
+
+
 def test_rms_numpy_fallback_large_array(benchmark):
     arr = np.arange(100_000_000, dtype=np.float32)
     rms = benchmark(rms_numpy, arr, window_size=5000)