--- a/dipy/tracking/distances.pyx
+++ b/dipy/tracking/distances.pyx
@@ -488,7 +488,7 @@ def bundles_distances_mam(tracksA, track
     # preprocess tracks
     cdef:
         size_t longest_track_len = 0, track_len
-        longest_track_lenA, longest_track_lenB
+        size_t longest_track_lenA = 0, longest_track_lenB = 0
         cnp.ndarray[object, ndim=1] tracksA32
         cnp.ndarray[object, ndim=1] tracksB32
         cnp.ndarray[cnp.double_t, ndim=2] DM
