diff --git a/lib/type_toolkit.rb b/lib/type_toolkit.rb
index 1994c4a..0162352 100644
--- a/lib/type_toolkit.rb
+++ b/lib/type_toolkit.rb
@@ -6,6 +6,7 @@
 require_relative "type_toolkit/ext/method"
 require_relative "type_toolkit/ext/module"
 require_relative "type_toolkit/ext/nil_assertions"
+require_relative "type_toolkit/ext/sorbet-runtime/nil_assertions"
 
 module TypeToolkit
 end
diff --git a/lib/type_toolkit/ext/nil_assertions.rb b/lib/type_toolkit/ext/nil_assertions.rb
index c444046..5543815 100644
--- a/lib/type_toolkit/ext/nil_assertions.rb
+++ b/lib/type_toolkit/ext/nil_assertions.rb
@@ -1,6 +1,8 @@
 # typed: true
 # frozen_string_literal: true
 
+require "bundler"
+
 # Asserts that the receiver is not nil.
 #
 # You should use `not_nil!` in places where you're absolutely sure a `nil` value can't occur.
@@ -18,7 +20,8 @@ class NilClass
   # @override
   #: -> bot
   def not_nil!
-    raise TypeToolkit::UnexpectedNilError
+    # Do not rely on this message content! Its content is subject to change.
+    raise TypeToolkit::UnexpectedNilError, "Called `not_nil!` on nil."
   end
 end
 
@@ -31,8 +34,5 @@ module TypeToolkit
   #
   # Note: `rescue Exception` can still catch it, but that's intentionally harder to write accidentally.
   class UnexpectedNilError < Exception # rubocop:disable Lint/InheritException
-    def initialize(message = "Called `not_nil!` on nil.")
-      super
-    end
   end
 end
diff --git a/lib/type_toolkit/ext/sorbet-runtime/nil_assertions.rb b/lib/type_toolkit/ext/sorbet-runtime/nil_assertions.rb
new file mode 100644
index 0000000..f9f4ccc
--- /dev/null
+++ b/lib/type_toolkit/ext/sorbet-runtime/nil_assertions.rb
@@ -0,0 +1,59 @@
+# typed: ignore
+# frozen_string_literal: true
+
+# This file is `typed: ignore` so we don't have to implement the abstract methods from `T::Types::Base`,
+# which `sorbet-runtime` will implement eventually anyway.
+
+require "bundler"
+return unless Bundler.locked_gems.specs.any? { |s| s.name == "sorbet-runtime" }
+
+module TypeToolkit
+  module SorbetRuntimeCompatibility
+    module VoidPatch
+      # An override of `not_nil!` intended to reduce the discrepancy between test and production environments
+      # when Sorbet Runtime is used in tests but not production.
+      #
+      # When Sorbet Runtime is active `void`-returning methods have their return value replaced with the
+      # `T::Private::Types::Void::VOID` module. If Sorbet Runtime is on in tests but not production,
+      # this introduces a dangerous difference in behaviour for methods that return `nil`:
+      #
+      # * In test code, it'll be replaced with `T::Private::Types::Void::VOID`.
+      #   If `not_nil!` is called on it (and this override didn't exist), it'll just return `self`.
+      #
+      # * In production code, that `nil` value will left-as-is, and calling `not_nil!` on it will raise an error.
+      #: -> bot
+      def not_nil!
+        # Do not rely on this message content! Its content is subject to change.
+        raise TypeToolkit::UnexpectedNilError, "Called `not_nil!` on a void value (T::Private::Types::Void::VOID)"
+      end
+    end
+  end
+end
+
+module T
+  module Types
+    class Base; end
+  end
+
+  module Private
+    module Types
+      class Void < T::Types::Base
+        if defined?(::T::Private::Types::Void::VOID)
+          # sorbet-runtime was already loaded, and the `VOID` module is already frozen, so we can't change it.
+          # Instead, replace it with our own copy.
+          ::T::Private::Types::Void.const_set(:VOID, ::Module.new do
+            extend ::TypeToolkit::SorbetRuntimeCompatibility::VoidPatch
+
+            freeze # Freeze it like the original
+          end)
+        else
+          # sorbet-runtime hasn't been loaded yet, so we're defining the `VOID` module for the first time.
+          module VOID
+            extend ::TypeToolkit::SorbetRuntimeCompatibility::VoidPatch
+            # Leave it unfrozen so sorbet-runtime can freeze it later when it defines it.
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/spec/nil_assertions_spec.rb b/spec/nil_assertions_spec.rb
index 711ae82..83fbfe1 100644
--- a/spec/nil_assertions_spec.rb
+++ b/spec/nil_assertions_spec.rb
@@ -2,10 +2,13 @@
 # frozen_string_literal: true
 
 require "spec_helper"
+require "sorbet-runtime"
 
 module TypeToolkit
   class NilAssertionsTest < Minitest::Spec
     describe "#not_nil!" do
+      extend T::Sig
+
       it "returns self on non-nil values" do
         x = "Hello, world!"
         assert_same x, x.not_nil!
@@ -14,6 +17,25 @@ class NilAssertionsTest < Minitest::Spec
       it "raises an error on nil values" do
         assert_raises(UnexpectedNilError) { nil.not_nil! }
       end
+
+      it "raises an error on Sorbet runtime's void value" do
+        # Cast away the void type to prevent the static type checker from blocking this with
+        # "Cannot call method `not_nil!` on void type"
+        void_value = example_void_returner #: as Object
+
+        assert_same T::Private::Types::Void::VOID, void_value
+        e = assert_raises(UnexpectedNilError) { void_value.not_nil! }
+
+        # Do not rely on this message content! Its content is subject to change.
+        assert_includes e.message, "Called `not_nil!` on a void value (T::Private::Types::Void::VOID)"
+      end
+
+      private
+
+      # To make sure we're patching the right thing, use a void-returning method to produce a void value,
+      # rather than directly referencing `T::Private::Types::Void::VOID`.
+      sig { void }
+      def example_void_returner = nil
     end
   end
 end