diff --git a/meegkit/asr.py b/meegkit/asr.py
index 8eec218b..f2a6c0d9 100755
--- a/meegkit/asr.py
+++ b/meegkit/asr.py
@@ -27,10 +27,6 @@ class ASR():
 
     Parameters
     ----------
-    X : array, shape=()
-        *zero-mean* (e.g., high-pass filtered) and reasonably clean EEG of not
-        much less than 30 seconds (this method is typically used with 1 minute
-        or more).
     sfreq : float
         Sampling rate of the data, in Hz.
 
@@ -162,7 +158,9 @@ def fit(self, X, y=None, **kwargs):
         ----------
         X : array, shape=(n_channels, n_samples)
             The calibration data should have been high-pass filtered (for
-            example at 0.5Hz or 1Hz using a Butterworth IIR filter).
+            example at 0.5Hz or 1Hz using a Butterworth IIR filter), and be
+            reasonably clean not less than 30 seconds (this method is typically
+            used with 1 minute or more).
 
         """
         if X.ndim == 3:
@@ -202,8 +200,6 @@ def transform(self, X, y=None, **kwargs):
         ----------
         X : array, shape=([n_trials, ]n_channels, n_samples)
             Raw data.
-        X_filt : array, shape=([n_trials, ]n_channels, n_samples)
-            Yulewalk-filtered data (optional). If this i
 
         Returns
         -------
@@ -218,7 +214,7 @@ def transform(self, X, y=None, **kwargs):
             else:
                 return X
 
-        # Yulewalk-filtered data (optional).
+        # Yulewalk-filtered data
         X_filt, self.zi_ = yulewalk_filter(
             X, sfreq=self.sfreq, ab=self.ab_, zi=self.zi_)
 
diff --git a/meegkit/utils/asr.py b/meegkit/utils/asr.py
index 8fe9e584..f93498af 100755
--- a/meegkit/utils/asr.py
+++ b/meegkit/utils/asr.py
@@ -316,7 +316,8 @@ def yulewalk_filter(X, sfreq, zi=None, ab=None, axis=-1):
     # apply the signal shaping filter and initialize the IIR filter state
     if zi is None:
         zi = signal.lfilter_zi(B, A)
-        out, zf = signal.lfilter(B, A, X, zi=zi[:, None] * X[:, 0], axis=axis)
+        zi = np.transpose(X[:, 0] * zi[:, None])
+        out, zf = signal.lfilter(B, A, X, zi=zi, axis=axis)
     else:
         out, zf = signal.lfilter(B, A, X, zi=zi, axis=axis)
 
diff --git a/tests/test_asr.py b/tests/test_asr.py
index b6995c32..8a0ac114 100644
--- a/tests/test_asr.py
+++ b/tests/test_asr.py
@@ -90,6 +90,31 @@ def test_yulewalk(sfreq, show=False):
         plt.show()
 
 
+@pytest.mark.parametrize(argnames='n_chans', argvalues=(4, 8, 12))
+def test_yulewalk_filter(n_chans, show=False):
+    """Test yulewalk filter."""
+    rawp = raw.copy()
+    n_chan_orig = rawp.shape[0]
+    rawp = np.random.randn(n_chans, n_chan_orig) @ rawp
+    raw_filt, iirstate = yulewalk_filter(rawp, sfreq)
+
+    if show:
+        f, ax = plt.subplots(n_chans, sharex=True, figsize=(8, 5))
+        for i in range(n_chans):
+            ax[i].plot(rawp[i], lw=.5, label='before')
+            ax[i].plot(raw_filt[i], label='after', lw=.5)
+            ax[i].set_ylim([-50, 50])
+            if i < n_chans - 1:
+                ax[i].set_yticks([])
+        ax[i].set_xlabel('Time (s)')
+        ax[i].set_ylabel(f'ch{i}')
+        ax[0].legend(fontsize='small', bbox_to_anchor=(1.04, 1),
+                     borderaxespad=0)
+        plt.subplots_adjust(hspace=0, right=0.75)
+        plt.suptitle('Before/after filter')
+        plt.show()
+
+
 def test_asr_functions(show=False, method='riemann'):
     """Test ASR functions (offline use).
 
@@ -178,3 +203,4 @@ def test_asr_class(show=False):
     # test_yulewalk(250, True)
     # test_asr_functions(True)
     # test_asr_class(True)
+    # test_yulewalk_filter(16, True)