diff --git a/heracles/healpy.py b/heracles/healpy.py
index 9f3759c..3efa709 100644
--- a/heracles/healpy.py
+++ b/heracles/healpy.py
@@ -178,7 +178,7 @@ def transform(self, data: NDArray[Any]) -> NDArray[Any]:
             msg = f"spin-{spin} maps not yet supported"
             raise NotImplementedError(msg)
 
-        alms = hp.map2alm(
+        alm = hp.map2alm(
             data,
             lmax=self.__lmax,
             pol=pol,
@@ -189,16 +189,16 @@ def transform(self, data: NDArray[Any]) -> NDArray[Any]:
         if pw is not None:
             fl = np.ones(self.__lmax + 1)
             fl[abs(spin) :] /= pw[abs(spin) :]
-            for alm in alms:
-                hp.almxfl(alm, fl, inplace=True)
+            for i in np.ndindex(*alm.shape[:-1]):
+                alm[i] = hp.almxfl(alm[i], fl)
             del fl
 
         if spin != 0:
-            alms = alms[1:].copy()
+            alm = alm[1:].copy()
 
-        update_metadata(alms, **md)
+        update_metadata(alm, **md)
 
-        return alms
+        return alm
 
     def resample(self, data: NDArray[Any]) -> NDArray[Any]:
         """
diff --git a/tests/test_healpy.py b/tests/test_healpy.py
index 77a6095..117662a 100644
--- a/tests/test_healpy.py
+++ b/tests/test_healpy.py
@@ -11,8 +11,10 @@
 else:
     HAVE_HEALPY = True
 
+skipif_not_healpy = pytest.mark.skipif(not HAVE_HEALPY, reason="test requires healpy")
 
-@pytest.mark.skipif(not HAVE_HEALPY, reason="test requires healpy")
+
+@skipif_not_healpy
 def test_healpix_maps(rng):
     from heracles.healpy import HealpixMapper
     from heracles.mapper import Mapper
@@ -76,7 +78,7 @@ def test_healpix_maps(rng):
     npt.assert_array_equal(m, expected)
 
 
-@pytest.mark.skipif(not HAVE_HEALPY, reason="test requires healpy")
+@skipif_not_healpy
 @unittest.mock.patch("healpy.map2alm")
 def test_healpix_transform(mock_map2alm, rng):
     from heracles.core import update_metadata
@@ -112,3 +114,50 @@ def test_healpix_transform(mock_map2alm, rng):
     assert alms.dtype.metadata["spin"] == 2
     assert alms.dtype.metadata["b"] == 2
     assert alms.dtype.metadata["nside"] == nside
+
+
+@skipif_not_healpy
+@unittest.mock.patch("healpy.map2alm")
+def test_healpix_deconvolve(mock_map2alm):
+    from heracles.core import update_metadata
+    from heracles.healpy import HealpixMapper
+
+    nside = 32
+    npix = 12 * nside**2
+
+    lmax = 48
+    nlm = (lmax + 1) * (lmax + 2) // 2
+
+    pw0, pw2 = hp.pixwin(nside, lmax=lmax, pol=True)
+    pw2[:2] = 1.0
+
+    mapper = HealpixMapper(nside, lmax, deconvolve=True)
+
+    # single scalar map
+    data = np.zeros(npix)
+    update_metadata(data, spin=0)
+
+    mock_map2alm.return_value = np.ones(nlm, dtype=complex)
+
+    alm = mapper.transform(data)
+
+    assert alm.shape == (nlm,)
+    stop = 0
+    for m in range(lmax + 1):
+        start, stop = stop, stop + lmax - m + 1
+        npt.assert_array_equal(alm[start:stop], 1.0 / pw0[m:])
+
+    # polarisation map
+    data = np.zeros((2, npix))
+    update_metadata(data, spin=2)
+
+    mock_map2alm.return_value = np.ones((3, nlm), dtype=complex)
+
+    alm = mapper.transform(data)
+
+    assert alm.shape == (2, nlm)
+    stop = 0
+    for m in range(lmax + 1):
+        start, stop = stop, stop + lmax - m + 1
+        npt.assert_array_equal(alm[0, start:stop], 1.0 / pw2[m:])
+        npt.assert_array_equal(alm[1, start:stop], 1.0 / pw2[m:])