diff --git a/skimage/filter/rank/core16_cy.pyx b/skimage/filter/rank/core16_cy.pyx
index 02acc93d..8fe72423 100644
--- a/skimage/filter/rank/core16_cy.pyx
+++ b/skimage/filter/rank/core16_cy.pyx
@@ -97,16 +97,16 @@ cdef void _core16(dtype_t kernel(Py_ssize_t*, float, dtype_t,
     # build attack and release borders
     # by using difference along axis
     t = np.hstack((selem, np.zeros((selem.shape[0], 1))))
-    t_e = np.diff(t, axis=1) < 0
+    cdef char[:, :] t_e = np.diff(t, axis=1) < 0
 
     t = np.hstack((np.zeros((selem.shape[0], 1)), selem))
-    t_w = np.diff(t, axis=1) > 0
+    cdef char[:, :] t_w = np.diff(t, axis=1) > 0
 
     t = np.vstack((selem, np.zeros((1, selem.shape[1]))))
-    t_s = np.diff(t, axis=0) < 0
+    cdef char[:, :] t_s = np.diff(t, axis=0) < 0
 
     t = np.vstack((np.zeros((1, selem.shape[1])), selem))
-    t_n = np.diff(t, axis=0) > 0
+    cdef char[:, :] t_n = np.diff(t, axis=0) > 0
 
     num_se_n = num_se_s = num_se_e = num_se_w = 0
 
diff --git a/skimage/filter/rank/core8_cy.pyx b/skimage/filter/rank/core8_cy.pyx
index 357a1051..0b6ac554 100644
--- a/skimage/filter/rank/core8_cy.pyx
+++ b/skimage/filter/rank/core8_cy.pyx
@@ -100,16 +100,16 @@ cdef void _core8(dtype_t kernel(Py_ssize_t*, float, dtype_t, float,
     # build attack and release borders
     # by using difference along axis
     t = np.hstack((selem, np.zeros((selem.shape[0], 1))))
-    t_e = np.diff(t, axis=1) < 0
+    cdef char[:, :] t_e = np.diff(t, axis=1) < 0
 
     t = np.hstack((np.zeros((selem.shape[0], 1)), selem))
-    t_w = np.diff(t, axis=1) > 0
+    cdef char[:, :] t_w = np.diff(t, axis=1) > 0
 
     t = np.vstack((selem, np.zeros((1, selem.shape[1]))))
-    t_s = np.diff(t, axis=0) < 0
+    cdef char[:, :] t_s = np.diff(t, axis=0) < 0
 
     t = np.vstack((np.zeros((1, selem.shape[1])), selem))
-    t_n = np.diff(t, axis=0) > 0
+    cdef char[:, :] t_n = np.diff(t, axis=0) > 0
 
     num_se_n = num_se_s = num_se_e = num_se_w = 0