diff --git a/CMakeLists.txt b/CMakeLists.txt
index c71fc9af1..332669d09 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -22,12 +22,12 @@ FetchContent_Declare(
 )
 # ... and Microsoft's C++ Guideline Support Library
 FetchContent_Declare(
-  GSL
+  Microsoft.GSL
   GIT_REPOSITORY https://github.com/microsoft/GSL
   GIT_TAG v4.2.0
   GIT_SHALLOW ON
   EXCLUDE_FROM_ALL # Do not install
-  FIND_PACKAGE_ARGS NAMES Microsoft.GSL
+  FIND_PACKAGE_ARGS
 )
 # ... and the Mimic C++ mocking framework
 FetchContent_Declare(
@@ -90,7 +90,7 @@ set(DART_TESTING_TIMEOUT 90 CACHE STRING "Timeout (s) for Dart/CTest runs")
 set(CTEST_TEST_TIMEOUT 90 CACHE STRING "Per-test timeout (s) for CTest")
 
 # Make tools available
-FetchContent_MakeAvailable(Catch2 GSL mimicpp)
+FetchContent_MakeAvailable(Catch2 Microsoft.GSL mimicpp)
 # Avoid use of __COUNTER__ to prevent -pedantic warnings in Catch2's TEST_CASE macro
 set(CATCH_CONFIG_COUNTER OFF)
 
@@ -163,6 +163,7 @@ find_package(Boost REQUIRED COMPONENTS json EXPORT)
 find_package(TBB REQUIRED EXPORT)
 find_package(fmt REQUIRED EXPORT)
 find_package(spdlog REQUIRED EXPORT)
+find_package(Microsoft.GSL REQUIRED EXPORT)
 
 # Dependencies required by internal implementation
 find_package(Boost REQUIRED COMPONENTS program_options)
diff --git a/phlex/driver.hpp b/phlex/driver.hpp
index 64c9d8c33..2e15673ae 100644
--- a/phlex/driver.hpp
+++ b/phlex/driver.hpp
@@ -12,7 +12,8 @@
 #include "boost/mp11/algorithm.hpp"
 #include "fmt/format.h"
 
-#include <cassert>
+#include <gsl/pointers>
+
 #include <concepts>
 #include <cstddef>
 #include <functional>
@@ -35,13 +36,13 @@ namespace phlex::detail {
     using driver_shim_t = void(driver_proxy const&, configuration const&, driver_bundle*);
 
     template <typename SourceType>
-    std::remove_cvref_t<SourceType> const& as_driver_source(source const* src, std::size_t index)
+    std::remove_cvref_t<SourceType> const& as_driver_source(gsl::not_null<source const*> src,
+                                                            std::size_t index)
     {
-      assert(src != nullptr);
-
       using expected_source_t = std::remove_cvref_t<SourceType>;
 
-      if (auto const* casted = dynamic_cast<expected_source_t const*>(src)) {
+      auto const* src_ptr = src.get();
+      if (auto const* casted = dynamic_cast<expected_source_t const*>(src_ptr)) {
         return *casted;
       }
 
@@ -49,7 +50,7 @@ namespace phlex::detail {
         fmt::format("Driver source type mismatch at source index {}: expected '{}' but got '{}'.",
                     index,
                     boost::core::demangle(typeid(expected_source_t).name()),
-                    boost::core::demangle(typeid(*src).name())));
+                    boost::core::demangle(typeid(*src_ptr).name())));
     }
 
     template <typename SourceParameters, typename F, typename FirstArg>
diff --git a/phlex/model/CMakeLists.txt b/phlex/model/CMakeLists.txt
index e9eb1ad5f..76d1fd776 100644
--- a/phlex/model/CMakeLists.txt
+++ b/phlex/model/CMakeLists.txt
@@ -20,6 +20,7 @@ cet_make_library(
   Boost::boost
   spdlog::spdlog
   fmt::fmt
+  Microsoft.GSL::GSL
   PRIVATE
   phlex::utilities
   TBB::tbb
diff --git a/phlex/model/fixed_hierarchy.hpp b/phlex/model/fixed_hierarchy.hpp
index 805d773be..24b6b8f47 100644
--- a/phlex/model/fixed_hierarchy.hpp
+++ b/phlex/model/fixed_hierarchy.hpp
@@ -6,6 +6,8 @@
 #include "phlex/model/fwd.hpp"
 #include "phlex/model/layer_path.hpp"
 
+#include <gsl/pointers>
+
 #include <cstddef>
 #include <initializer_list>
 #include <string>
@@ -37,8 +39,8 @@ namespace phlex {
     data_cell_index_ptr index_;
     // Non-owning pointers to the enclosing hierarchy and driver; data_cell_cursor is a
     // short-lived, freely copyable view and does not manage their lifetimes.
-    fixed_hierarchy const* hierarchy_;
-    detail::framework_driver* driver_;
+    gsl::not_null<fixed_hierarchy const*> hierarchy_;
+    gsl::not_null<detail::framework_driver*> driver_;
   };
 
   /// @brief Callable object that yields data cells to the framework driver.
diff --git a/phlex/model/handle.hpp b/phlex/model/handle.hpp
index b807b54a0..551950cf0 100644
--- a/phlex/model/handle.hpp
+++ b/phlex/model/handle.hpp
@@ -5,6 +5,8 @@
 #include "phlex/model/fwd.hpp"
 #include "phlex/model/product_specification.hpp"
 
+#include <gsl/pointers>
+
 #include <optional>
 #include <type_traits>
 #include <utility>
@@ -83,7 +85,7 @@ namespace phlex {
     handle(handle&&) noexcept = default;
     handle& operator=(handle&&) noexcept = default;
 
-    const_pointer operator->() const noexcept { return product_; }
+    const_pointer operator->() const noexcept { return product_.get(); }
     [[nodiscard]] const_reference operator*() const noexcept { return *operator->(); }
     // NOLINTBEGIN(google-explicit-constructor) - Implicit conversion is intentional
     operator const_reference() const noexcept { return operator*(); }
@@ -116,8 +118,10 @@ namespace phlex {
     }
 
   private:
-    const_pointer product_;           // Non-null, by construction
-    class data_cell_index const* id_; // Non-null, by construction
+    // Non-owning pointers to the product and its data-cell index; both are non-null by
+    // construction, as enforced by gsl::not_null.
+    gsl::not_null<const_pointer> product_;
+    gsl::not_null<class data_cell_index const*> id_;
     experimental::identifier creator_plugin_;
     experimental::identifier creator_algorithm_;
     experimental::identifier suffix_;
diff --git a/phlex/model/products.cpp b/phlex/model/products.cpp
index 12953f894..b10ba158e 100644
--- a/phlex/model/products.cpp
+++ b/phlex/model/products.cpp
@@ -17,7 +17,7 @@ namespace phlex::detail {
   products::size_type products::size() const noexcept { return products_.size(); }
   bool products::empty() const noexcept { return products_.empty(); }
 
-  product_base const* products::find_product(product_specification const& spec) const
+  gsl::not_null<product_base const*> products::find_product(product_specification const& spec) const
   {
     auto it =
       std::ranges::find(products_, spec, [](auto const& p) -> auto const& { return p.first; });
diff --git a/phlex/model/products.hpp b/phlex/model/products.hpp
index 2732a09f7..fd664686d 100644
--- a/phlex/model/products.hpp
+++ b/phlex/model/products.hpp
@@ -5,9 +5,12 @@
 
 #include "phlex/model/product_specification.hpp"
 
+#include <gsl/pointers>
+
 #include <cassert>
 #include <concepts>
 #include <memory>
+#include <stdexcept>
 #include <string>
 #include <typeinfo>
 #include <utility>
@@ -46,7 +49,10 @@ namespace phlex::detail {
   product_ptr product_for(T&& t)
   {
     if constexpr (std::convertible_to<T, product_ptr>) {
-      return std::forward<T>(t);
+      if (auto product = product_ptr{std::forward<T>(t)}) {
+        return product;
+      }
+      throw std::runtime_error("Cannot store a null product pointer.");
     } else {
       return std::make_unique<product<std::remove_cvref_t<T>>>(std::forward<T>(t));
     }
@@ -87,9 +93,9 @@ namespace phlex::detail {
     template <typename T>
     T const& get(product_specification const& spec) const
     {
-      auto const* available_product = find_product(spec);
+      auto const available_product = find_product(spec);
 
-      if (auto const* desired_product = dynamic_cast<product<T> const*>(available_product)) {
+      if (auto const* desired_product = dynamic_cast<product<T> const*>(available_product.get())) {
         return desired_product->obj;
       }
 
@@ -102,7 +108,9 @@ namespace phlex::detail {
     bool empty() const noexcept;
 
   private:
-    product_base const* find_product(product_specification const& spec) const;
+    // Throws if no product matches the specification; the returned pointer is therefore
+    // never null.
+    gsl::not_null<product_base const*> find_product(product_specification const& spec) const;
     static void throw_mismatched_type [[noreturn]] (product_specification const& spec,
                                                     char const* requested_type,
                                                     char const* available_type);
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index bb5197188..1ac190098 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -191,6 +191,9 @@ cet_test(product_handle USE_CATCH2_MAIN SOURCE product_handle_test.cpp LIBRARIES
 cet_test(product_store USE_CATCH2_MAIN SOURCE product_store_test.cpp LIBRARIES
          phlex::core_internal
 )
+cet_test(products USE_CATCH2_MAIN SOURCE products_test.cpp LIBRARIES
+         phlex::model_internal
+)
 cet_test(
   fold
   USE_CATCH2_MAIN
diff --git a/test/products_test.cpp b/test/products_test.cpp
new file mode 100644
index 000000000..ee9c7b34d
--- /dev/null
+++ b/test/products_test.cpp
@@ -0,0 +1,11 @@
+#include "phlex/model/products.hpp"
+
+#include "catch2/catch_test_macros.hpp"
+#include "catch2/matchers/catch_matchers.hpp"
+
+TEST_CASE("Reject null product pointers", "[data model]")
+{
+  using namespace phlex::detail;
+  CHECK_THROWS_WITH(product_for(product_ptr{}), "Cannot store a null product pointer.");
+  CHECK_THROWS_WITH(product_for(nullptr), "Cannot store a null product pointer.");
+}