diff --git a/cpp/src/unary/cast_ops.cu b/cpp/src/unary/cast_ops.cu
index fcee2b368fcc..49fbbf76e663 100644
--- a/cpp/src/unary/cast_ops.cu
+++ b/cpp/src/unary/cast_ops.cu
@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: Copyright (c) 2019-2026, NVIDIA CORPORATION.
+ * SPDX-FileCopyrightText: Copyright (c) 2019-2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
  * SPDX-License-Identifier: Apache-2.0
  */
 
@@ -118,7 +118,7 @@ struct fixed_point_unary_cast {
 template <typename From, typename To>
 constexpr inline auto is_supported_non_fixed_point_cast()
 {
-  return cudf::is_fixed_width<To>() &&
+  return cudf::is_fixed_width<From>() && cudf::is_fixed_width<To>() &&
          // Disallow fixed_point here (requires different specialization)
          !(cudf::is_fixed_point<From>() || cudf::is_fixed_point<To>()) &&
          // Disallow conversions between timestamps and numeric
diff --git a/cpp/tests/unary/cast_tests.cpp b/cpp/tests/unary/cast_tests.cpp
index 5fcec956a8d9..1fa8a825670d 100644
--- a/cpp/tests/unary/cast_tests.cpp
+++ b/cpp/tests/unary/cast_tests.cpp
@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: Copyright (c) 2020-2026, NVIDIA CORPORATION.
+ * SPDX-FileCopyrightText: Copyright (c) 2020-2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
  * SPDX-License-Identifier: Apache-2.0
  */
 
@@ -201,6 +201,12 @@ inline auto make_data_type()
   return cudf::data_type{cudf::type_to_id<T>()};
 }
 
+TEST(IsSupportedCast, StringToInt32IsUnsupported)
+{
+  EXPECT_FALSE(cudf::is_supported_cast(cudf::data_type{cudf::type_id::STRING},
+                                       cudf::data_type{cudf::type_id::INT32}));
+}
+
 struct CastTimestampsSimple : public cudf::test::BaseFixture {};
 
 TEST_F(CastTimestampsSimple, IsIdempotent)