diff --git a/skimage/segmentation/_slic.pyx b/skimage/segmentation/_slic.pyx
index 98a6f6bc..684740d6 100644
--- a/skimage/segmentation/_slic.pyx
+++ b/skimage/segmentation/_slic.pyx
@@ -51,9 +51,10 @@ def slic(image, n_segments=100, ratio=10., max_iter=10, sigma=1,
         image = rgb2lab(image)
 
     # initialize on grid:
+    cdef int height, width
     height, width = image.shape[:2]
     # approximate grid size for desired n_segments
-    step = np.ceil(np.sqrt(height * width / n_segments))
+    cdef int step = np.ceil(np.sqrt(height * width / n_segments))
     grid_y, grid_x = np.mgrid[:height, :width]
     means_y = grid_y[::step, ::step]
     means_x = grid_x[::step, ::step]