diff --git a/pandas/_libs/algos.pyx b/pandas/_libs/algos.pyx
index b54ab9030e4a1..ed251c401c277 100644
--- a/pandas/_libs/algos.pyx
+++ b/pandas/_libs/algos.pyx
@@ -1371,8 +1371,8 @@ ctypedef fused out_t:
 @cython.boundscheck(False)
 @cython.wraparound(False)
 def diff_2d(
-    # TODO: cython bug (post Cython 3) preventsupdate to "const diff_t[:, :] arr"
-    diff_t[:, :] arr,
+    # TODO: cython bug (post Cython 3) prevents update to "const diff_t[:, :] arr"
+    ndarray[diff_t, ndim=2] arr,
     out_t[:, :] out,
     Py_ssize_t periods,
     int axis,
@@ -1380,7 +1380,9 @@ def diff_2d(
 ):
     cdef:
         Py_ssize_t i, j, sx, sy, start, stop
-        bint f_contig = arr.is_f_contig()
+        bint f_contig = arr.flags.f_contiguous
+        # TODO: change to this when arr becomes a memoryview
+        # bint f_contig = arr.is_f_contig()
         diff_t left, right
 
     # Disable for unsupported dtype combinations,
diff --git a/pandas/_libs/groupby.pyx b/pandas/_libs/groupby.pyx
index 1d63030de026e..3384060f74c20 100644
--- a/pandas/_libs/groupby.pyx
+++ b/pandas/_libs/groupby.pyx
@@ -1545,7 +1545,12 @@ def group_nth(
                 if uses_mask:
                     isna_entry = mask[i, j]
                 else:
-                    isna_entry = _treat_as_na(val, is_datetimelike)
+                    # TODO: just make _treat_as_na support this?
+                    # remove notimplemented for object dtype there
+                    if numeric_object_t is object:
+                        isna_entry = checknull(val)
+                    else:
+                        isna_entry = _treat_as_na(val, is_datetimelike)
 
                 if not isna_entry:
                     nobs[lab, j] += 1