diff --git a/brainles_preprocessing/defacing/quickshear/nipy_quickshear.py b/brainles_preprocessing/defacing/quickshear/nipy_quickshear.py
index 6654eaa..2123887 100644
--- a/brainles_preprocessing/defacing/quickshear/nipy_quickshear.py
+++ b/brainles_preprocessing/defacing/quickshear/nipy_quickshear.py
@@ -155,11 +155,16 @@ def run_quickshear(bet_img: nb.nifti1.Nifti1Image, buffer: int = 10) -> NDArray:
     slope = ydiffs[0] / xdiffs[0]
 
     yint = low[1][0] - (low[0][0] * slope) - buffer
-    ys = np.arange(0, mask_RPS.shape[2]) * slope + yint
+    ys = np.arange(0, mask_RPS.shape[1]) * slope + yint
     defaced_mask_RPS = np.ones(mask_RPS.shape, dtype="bool")
 
-    for x, y in zip(np.nonzero(ys > 0)[0], ys.astype(int)):
-        defaced_mask_RPS[:, x, :y] = 0
+    for x in np.nonzero(ys > 0)[0]:
+        # Read the cutoff from ys[x]: zipping the filtered indices with the
+        # unfiltered ys array pairs each position with the wrong cutoff
+        # whenever ys starts with a nonpositive prefix, and a negative cutoff
+        # would make the slice erase the wrong region.
+        y_clipped = min(int(ys[x]), mask_RPS.shape[2])
+        defaced_mask_RPS[:, x, :y_clipped] = 0
 
     defaced_mask = nb.orientations.apply_orientation(defaced_mask_RPS, from_RPS)