diff --git a/Chapter07/w1_unwrapping_image_exercise.py b/Chapter07/w1_unwrapping_image_exercise.py
index e5f5b2a057c64b09b5ac45ad9d05b00291341110..8c7c5ec54a2890af279f15686d4e80eb82b2ead6 100644
--- a/Chapter07/w1_unwrapping_image_exercise.py
+++ b/Chapter07/w1_unwrapping_image_exercise.py
@@ -28,7 +28,7 @@ Y = center[1] + np.outer(radii,np.sin(angles))
 
 F = scipy.interpolate.interp2d(np.arange(I.shape[0]), np.arange(I.shape[1]), I)
 U = np.array([F(p[0],p[1]) for p in np.c_[Y.ravel(),X.ravel()]])
-U = U.reshape((r,a)).astype(np.uint8)
+U = U.reshape((r,a)).astype(np.int32)
 
 fig, ax = plt.subplots(1,2)
 ax[0].imshow(I, cmap='gray')