diff --git a/awkward-cpp/src/cpu-kernels/awkward_UnionArray_regular_index.cpp b/awkward-cpp/src/cpu-kernels/awkward_UnionArray_regular_index.cpp
index bd7deb8c56..362800cf73 100644
--- a/awkward-cpp/src/cpu-kernels/awkward_UnionArray_regular_index.cpp
+++ b/awkward-cpp/src/cpu-kernels/awkward_UnionArray_regular_index.cpp
@@ -21,6 +21,45 @@ ERROR awkward_UnionArray_regular_index(
   }
   return success();
 }
+ERROR awkward_UnionArray64_32_regular_index(
+  int32_t* toindex,
+  int32_t* current,
+  int64_t size,
+  const int64_t* fromtags,
+  int64_t length) {
+  return awkward_UnionArray_regular_index<int64_t, int32_t>(
+    toindex,
+    current,
+    size,
+    fromtags,
+    length);
+}
+ERROR awkward_UnionArray64_U32_regular_index(
+  uint32_t* toindex,
+  uint32_t* current,
+  int64_t size,
+  const int64_t* fromtags,
+  int64_t length) {
+  return awkward_UnionArray_regular_index<int64_t, uint32_t>(
+    toindex,
+    current,
+    size,
+    fromtags,
+    length);
+}
+ERROR awkward_UnionArray64_64_regular_index(
+  int64_t* toindex,
+  int64_t* current,
+  int64_t size,
+  const int64_t* fromtags,
+  int64_t length) {
+  return awkward_UnionArray_regular_index<int64_t, int64_t>(
+    toindex,
+    current,
+    size,
+    fromtags,
+    length);
+}
 ERROR awkward_UnionArray8_32_regular_index(
   int32_t* toindex,
   int32_t* current,
diff --git a/awkward-cpp/src/cpu-kernels/awkward_UnionArray_regular_index_getsize.cpp b/awkward-cpp/src/cpu-kernels/awkward_UnionArray_regular_index_getsize.cpp
index f660fd8343..88d40fa49a 100644
--- a/awkward-cpp/src/cpu-kernels/awkward_UnionArray_regular_index_getsize.cpp
+++ b/awkward-cpp/src/cpu-kernels/awkward_UnionArray_regular_index_getsize.cpp
@@ -19,6 +19,15 @@ ERROR awkward_UnionArray_regular_index_getsize(
   *size = *size + 1;
   return success();
 }
+ERROR awkward_UnionArray64_regular_index_getsize(
+  int64_t* size,
+  const int64_t* fromtags,
+  int64_t length) {
+  return awkward_UnionArray_regular_index_getsize<int64_t>(
+    size,
+    fromtags,
+    length);
+}
 ERROR awkward_UnionArray8_regular_index_getsize(
   int64_t* size,
   const int8_t* fromtags,
diff --git a/kernel-specification.yml b/kernel-specification.yml
index 5c901f00e2..d95b40a01d 100644
--- a/kernel-specification.yml
+++ b/kernel-specification.yml
@@ -3458,6 +3458,27 @@ kernels:
 
   - name: awkward_UnionArray_regular_index
     specializations:
+      - name: awkward_UnionArray64_32_regular_index
+        args:
+          - {name: toindex, type: "List[int32_t]", dir: out}
+          - {name: current, type: "List[int32_t]", dir: out}
+          - {name: size, type: "int64_t", dir: in, role: default}
+          - {name: fromtags, type: "Const[List[int64_t]]", dir: in, role: UnionArray-tags}
+          - {name: length, type: "int64_t", dir: in, role: default}
+      - name: awkward_UnionArray64_64_regular_index
+        args:
+          - {name: toindex, type: "List[int64_t]", dir: out}
+          - {name: current, type: "List[int64_t]", dir: out}
+          - {name: size, type: "int64_t", dir: in, role: default}
+          - {name: fromtags, type: "Const[List[int64_t]]", dir: in, role: UnionArray-tags}
+          - {name: length, type: "int64_t", dir: in, role: default}
+      - name: awkward_UnionArray64_U32_regular_index
+        args:
+          - {name: toindex, type: "List[uint32_t]", dir: out}
+          - {name: current, type: "List[uint32_t]", dir: out}
+          - {name: size, type: "int64_t", dir: in, role: default}
+          - {name: fromtags, type: "Const[List[int64_t]]", dir: in, role: UnionArray-tags}
+          - {name: length, type: "int64_t", dir: in, role: default}
       - name: awkward_UnionArray8_32_regular_index
         args:
           - {name: toindex, type: "List[int32_t]", dir: out}
@@ -3493,6 +3514,11 @@ kernels:
 
   - name: awkward_UnionArray_regular_index_getsize
     specializations:
+      - name: awkward_UnionArray64_regular_index_getsize
+        args:
+          - {name: size, type: "List[int64_t]", dir: out}
+          - {name: fromtags, type: "Const[List[int64_t]]", dir: in, role: UnionArray-tags}
+          - {name: length, type: "int64_t", dir: in, role: default}
       - name: awkward_UnionArray8_regular_index_getsize
         args:
           - {name: size, type: "List[int64_t]", dir: out}
diff --git a/tests/test_3312_concatenate_regular_arrays.py b/tests/test_3312_concatenate_regular_arrays.py
new file mode 100644
index 0000000000..79b8947a6f
--- /dev/null
+++ b/tests/test_3312_concatenate_regular_arrays.py
@@ -0,0 +1,11 @@
+from __future__ import annotations
+
+import numpy as np
+
+import awkward as ak
+
+
+def test():
+    ak.concatenate([ak.Array([i])[:, np.newaxis] for i in range(127)], axis=1)
+
+    ak.concatenate([ak.Array([i])[:, np.newaxis] for i in range(128)], axis=1)