diff --git a/detectron/utils/blob.py b/detectron/utils/blob.py
index b4fa50c9d..59cecc519 100644
--- a/detectron/utils/blob.py
+++ b/detectron/utils/blob.py
@@ -106,7 +106,7 @@ def prep_im_for_blob(im, pixel_means, target_size, max_size):
     the scale factors that were used to compute each returned image.
     """
     im = im.astype(np.float32, copy=False)
-    im -= pixel_means
+    im = cv2.subtract(im, pixel_means.reshape((1,) + im.shape[-1:]))
     im_shape = im.shape
     im_size_min = np.min(im_shape[0:2])
     im_size_max = np.max(im_shape[0:2])