diff --git a/cpp/include/cudf/detail/null_mask.hpp b/cpp/include/cudf/detail/null_mask.hpp
index 6ee406de5ef..83ef78a8250 100644
--- a/cpp/include/cudf/detail/null_mask.hpp
+++ b/cpp/include/cudf/detail/null_mask.hpp
@@ -268,9 +268,9 @@ std::pair<rmm::device_buffer, size_type> bitmask_or(
  * @param mask_size_bits The number of bits to be ANDed in each mask
  * @param stream CUDA stream used for device memory operations and kernel launches
  * @param mr Device memory resource used to allocate the returned device_buffer
- * @return rmm::device_buffer Output bitmask
+ * @return Count of set bits
  */
-void inplace_bitmask_and(
+cudf::size_type inplace_bitmask_and(
   device_span<bitmask_type> dest_mask,
   host_span<bitmask_type const*> masks,
   host_span<size_type const> masks_begin_bits,
diff --git a/cpp/src/bitmask/null_mask.cu b/cpp/src/bitmask/null_mask.cu
index ec3776fb6d5..d1107ad3cfd 100644
--- a/cpp/src/bitmask/null_mask.cu
+++ b/cpp/src/bitmask/null_mask.cu
@@ -404,14 +404,14 @@ std::vector<size_type> segmented_null_count(const bitmask_type* bitmask,
 }
 
 // Inplace Bitwise AND of the masks
-void inplace_bitmask_and(device_span<bitmask_type> dest_mask,
-                         host_span<bitmask_type const*> masks,
-                         host_span<size_type const> begin_bits,
-                         size_type mask_size,
-                         rmm::cuda_stream_view stream,
-                         rmm::mr::device_memory_resource* mr)
+cudf::size_type inplace_bitmask_and(device_span<bitmask_type> dest_mask,
+                                    host_span<bitmask_type const*> masks,
+                                    host_span<size_type const> begin_bits,
+                                    size_type mask_size,
+                                    rmm::cuda_stream_view stream,
+                                    rmm::mr::device_memory_resource* mr)
 {
-  inplace_bitmask_binop(
+  return inplace_bitmask_binop(
     [] __device__(bitmask_type left, bitmask_type right) { return left & right; },
     dest_mask,
     masks,
diff --git a/cpp/src/structs/utilities.cpp b/cpp/src/structs/utilities.cpp
index 43a32c8405a..afea8a55b16 100644
--- a/cpp/src/structs/utilities.cpp
+++ b/cpp/src/structs/utilities.cpp
@@ -322,14 +322,15 @@ void superimpose_parent_nulls(bitmask_type const* parent_null_mask,
       reinterpret_cast<bitmask_type const*>(parent_null_mask),
       reinterpret_cast<bitmask_type const*>(current_child_mask)};
     std::vector<size_type> begin_bits{0, 0};
-    cudf::detail::inplace_bitmask_and(
+    auto const valid_count = cudf::detail::inplace_bitmask_and(
       device_span<bitmask_type>(current_child_mask, num_bitmask_words(child.size())),
       masks,
       begin_bits,
       child.size(),
       stream,
       mr);
-    child.set_null_count(UNKNOWN_NULL_COUNT);
+    auto const null_count = child.size() - valid_count;
+    child.set_null_count(null_count);
   }
 
   // If the child is also a struct, repeat for all grandchildren.