diff --git a/inferelator_velocity/utils/mcv.py b/inferelator_velocity/utils/mcv.py
index ea3229f..f43e549 100644
--- a/inferelator_velocity/utils/mcv.py
+++ b/inferelator_velocity/utils/mcv.py
@@ -118,11 +118,11 @@ def _molecular_split(count_data, random_seed=800, p=0.5):
 
     rng = np.random.default_rng(random_seed)
 
-    if sps.issparse(count_data):
+    normalization_depth = np.median(
+        safer_sum(count_data, 1)
+    )
 
-        normalization_depth = np.median(
-            count_data.sum(axis=1).A1
-        )
+    if sps.issparse(count_data):
 
         if sps.isspmatrix_csr(count_data):
             mat_func = sps.csr_matrix
@@ -145,10 +145,6 @@ def _molecular_split(count_data, random_seed=800, p=0.5):
 
     else:
 
-        normalization_depth = np.median(
-            count_data.sum(axis=1)
-        )
-
         cv_data = np.zeros_like(count_data)
 
         for i in range(count_data.shape[0]):
@@ -214,6 +210,26 @@ def _mse_rowwise(
 
         return output
 
+    @numba.njit(parallel=False)
+    def _sum_columns(data, indices, n_col):
+
+        output = np.zeros(n_col, dtype=data.dtype)
+
+        for i in numba.prange(data.shape[0]):
+            output[indices[i]] += data[i]
+
+        return output
+
+    @numba.njit(parallel=False)
+    def _sum_rows(data, indptr):
+
+        output = np.zeros(indptr.shape[0] - 1, dtype=data.dtype)
+
+        for i in numba.prange(output.shape[0]):
+            output[i] = np.sum(data[indptr[i]:indptr[i + 1]])
+
+        return output
+
     def mcv_mse(x, pc, rotation, by_row=False, **metric_kwargs):
 
         if sps.issparse(x):
@@ -242,6 +258,27 @@ def mcv_mse(x, pc, rotation, by_row=False, **metric_kwargs):
                 **metric_kwargs
             )
 
+    def safer_sum(sparse_array, axis=None):
+
+        if not sps.issparse(sparse_array):
+            return np.sum(sparse_array, axis=axis)
+
+        if axis is None:
+            return np.sum(sparse_array.data)
+
+        elif axis == 0:
+            return _sum_columns(
+                sparse_array.data,
+                sparse_array.indices,
+                sparse_array.shape[1]
+            )
+
+        elif axis == 1:
+            return _sum_rows(
+                sparse_array.data,
+                sparse_array.indptr
+            )
+
 except ImportError:
 
     def mcv_mse(x, pc, rotation, **metric_kwargs):
@@ -258,3 +295,12 @@ def mcv_mse(x, pc, rotation, **metric_kwargs):
             metric='mse',
             **metric_kwargs
         )
+
+    def safer_sum(sparse_array, axis=None):
+
+        x = sparse_array.sum(axis)
+
+        if hasattr(x, 'A1'):
+            return x.A1
+        else:
+            return x