diff --git a/cpp/src/arrow/array/concatenate.cc b/cpp/src/arrow/array/concatenate.cc
index d96dce4d14675..b4638dd6593d8 100644
--- a/cpp/src/arrow/array/concatenate.cc
+++ b/cpp/src/arrow/array/concatenate.cc
@@ -921,6 +921,8 @@ namespace internal {
 Result<std::shared_ptr<Array>> Concatenate(
     const ArrayVector& arrays, MemoryPool* pool,
     std::shared_ptr<DataType>* out_suggested_cast) {
+  DCHECK(out_suggested_cast);
+  *out_suggested_cast = nullptr;
   if (arrays.size() == 0) {
     return Status::Invalid("Must pass at least one array");
   }
diff --git a/cpp/src/arrow/array/concatenate.h b/cpp/src/arrow/array/concatenate.h
index c22f3043ec3f6..aada5624d63a3 100644
--- a/cpp/src/arrow/array/concatenate.h
+++ b/cpp/src/arrow/array/concatenate.h
@@ -30,10 +30,9 @@ namespace internal {
 ///
 /// \param[in] arrays a vector of arrays to be concatenated
 /// \param[in] pool memory to store the result will be allocated from this memory pool
-/// \param[out] out_suggested_cast if non-NULL, the function might set it to a cast
-///                                suggestion that would allow concatenating the arrays
-///                                without overflow of offsets (e.g. string to
-///                                large_string)
+/// \param[out] out_suggested_cast if a non-OK Result is returned, the function might set
+///   out_suggested_cast to a cast suggestion that would allow concatenating the arrays
+///   without overflow of offsets (e.g. string to large_string)
 ///
 /// \return the concatenated array
 ARROW_EXPORT