diff --git a/package/MDAnalysis/lib/_cutil.pyx b/package/MDAnalysis/lib/_cutil.pyx
index a342650dec7..856d3f4a06e 100644
--- a/package/MDAnalysis/lib/_cutil.pyx
+++ b/package/MDAnalysis/lib/_cutil.pyx
@@ -30,7 +30,7 @@ __all__ = ['unique_int_1d', ]
 
 @cython.boundscheck(False) # turn off bounds-checking for entire function
 @cython.wraparound(False)  # turn off negative index wrapping for entire function
-def unique_int_1d(np.ndarray[np.int_t, ndim=1] values):
+def unique_int_1d(np.ndarray[np.int64_t, ndim=1] values):
     """
     Find the unique elements of a 1D array of integers.
 
@@ -51,7 +51,7 @@ def unique_int_1d(np.ndarray[np.int_t, ndim=1] values):
     cdef int i = 0
     cdef int j = 0
     cdef int n_values = values.shape[0]
-    cdef np.ndarray[np.int_t, ndim=1] result = np.empty(n_values, dtype=int)
+    cdef np.ndarray[np.int64_t, ndim=1] result = np.empty(n_values, dtype=np.int64)
 
     if n_values == 0:
         return result