diff --git a/docs/src/python/cluster/pyClusterVector.rst b/docs/src/python/cluster/pyClusterVector.rst
index 5805fcd8..6778278f 100644
--- a/docs/src/python/cluster/pyClusterVector.rst
+++ b/docs/src/python/cluster/pyClusterVector.rst
@@ -72,10 +72,10 @@ API.
 .. py:function:: reduce_to_3x3(clustervector)
    :noindex:
 
-   Return a new vector containing the center adjacent highest sum 3x3 block of every input cluster.
+   Return a new vector containing the central 3x3 block of every input cluster.
    Cluster order, coordinates, frame number, and pixel dtype are preserved.
 
-   :param ClusterVector clustervector: Input clusters of size 3x3 or larger.
+   :param ClusterVector clustervector: Input clusters with odd dimensions of 3x3 or larger.
    :return: Reduced 3x3 clusters.
    :rtype: ClusterVector
 
diff --git a/include/aare/Cluster.hpp b/include/aare/Cluster.hpp
index 061b0b2d..b2a274fe 100755
--- a/include/aare/Cluster.hpp
+++ b/include/aare/Cluster.hpp
@@ -193,7 +193,8 @@ Cluster<T, 2, 2, uint16_t> reduce_to_2x2(const Cluster<T, 3, 3, uint16_t> &c) {
 /**
  * @brief Reduce a cluster to the 3x3 block around its center index.
  * @param c Cluster to reduce
- * @return Reduced cluster with the input coordinates
+ * @pre ClusterSizeX and ClusterSizeY must both be odd.
+ * @return Reduced cluster. Input coordinates are preserved.
  */
 template <typename T, uint8_t ClusterSizeX, uint8_t ClusterSizeY,
           typename CoordType = int16_t>
@@ -202,6 +203,8 @@ reduce_to_3x3(const Cluster<T, ClusterSizeX, ClusterSizeY, CoordType> &c) {
 
     static_assert(ClusterSizeX >= 3 && ClusterSizeY >= 3,
                   "Cluster sizes must be at least 3x3 for reduction to 3x3");
+    static_assert(ClusterSizeX % 2 == 1 && ClusterSizeY % 2 == 1,
+                  "Cluster sizes must be odd for reduction to 3x3");
 
     Cluster<T, 3, 3, CoordType> result{};
 
diff --git a/include/aare/ClusterVector.hpp b/include/aare/ClusterVector.hpp
index 9b045d8c..65246ff1 100644
--- a/include/aare/ClusterVector.hpp
+++ b/include/aare/ClusterVector.hpp
@@ -241,6 +241,7 @@ ClusterVector<Cluster<T, 2, 2, CoordType>> reduce_to_2x2(
 /**
  * @brief Reduce every cluster to the 3x3 block around its center index.
  * @param cv ClusterVector containing clusters to reduce
+ * @pre ClusterSizeX and ClusterSizeY must both be odd.
  * @return ClusterVector of 3x3 clusters in the original order and with the
  * original frame number
  * @note Coordinates are preserved.
diff --git a/python/src/bind_Cluster.hpp b/python/src/bind_Cluster.hpp
index 2ff37270..f97f42f6 100644
--- a/python/src/bind_Cluster.hpp
+++ b/python/src/bind_Cluster.hpp
@@ -91,6 +91,7 @@ void reduce_to_3x3(py::module &m) {
         py::return_value_policy::move, py::arg("cluster"), R"doc(
         Return the 3x3 block around the cluster's center index.
 
+        The input cluster dimensions must both be odd.
         The input coordinates are preserved and output data is stored in
         row-major order.
         )doc");
diff --git a/python/src/bind_ClusterVector.hpp b/python/src/bind_ClusterVector.hpp
index 2780eeea..03e0a57b 100644
--- a/python/src/bind_ClusterVector.hpp
+++ b/python/src/bind_ClusterVector.hpp
@@ -225,6 +225,7 @@ void define_3x3_reduction(py::module &m) {
         },
         R"doc(
         Reduce every cluster to the 3x3 block around its center index.
+        The input cluster dimensions must both be odd.
         Returns a new ClusterVector; cluster order, coordinates, and frame
         number are preserved.
         )doc",