diff --git a/checker/internal/type_checker_builder_impl.cc b/checker/internal/type_checker_builder_impl.cc
index ea869056d..ace6afd92 100644
--- a/checker/internal/type_checker_builder_impl.cc
+++ b/checker/internal/type_checker_builder_impl.cc
@@ -175,6 +175,14 @@ absl::Status ValidateType(const Type& t, bool check_type_param_name,
       return ValidateType(value_type, check_type_param_name, depth_limit,
                           remaining_depth);
     }
+    case TypeKind::kStruct: {
+      auto message_type = t.AsMessage();
+      if (message_type.has_value() && !static_cast<bool>(*message_type)) {
+        return absl::InvalidArgumentError(
+            "an empty message type cannot be used in a type declaration");
+      }
+      return absl::OkStatus();
+    }
     case TypeKind::kOpaque: {
       for (Type type_param : t.AsOpaque()->GetParameters()) {
         CEL_RETURN_IF_ERROR(ValidateType(type_param, check_type_param_name,
diff --git a/checker/internal/type_checker_builder_impl_test.cc b/checker/internal/type_checker_builder_impl_test.cc
index 6b3e7a890..5e4054128 100644
--- a/checker/internal/type_checker_builder_impl_test.cc
+++ b/checker/internal/type_checker_builder_impl_test.cc
@@ -207,7 +207,8 @@ TEST(ContextDeclsTest, ErrorOnOverlappingVariableDeclaration) {
                        "variable 'single_int64' declared multiple times"));
 }
 
-TEST(ContextDeclsTest, InvalidTypeParamNameVariableValidationDisabled) {
+TEST(TypeCheckerBuilderImplTest,
+     InvalidTypeParamNameVariableValidationDisabled) {
   CheckerOptions options;
   options.enable_type_parameter_name_validation = false;
   TypeCheckerBuilderImpl builder(internal::GetSharedTestingDescriptorPool(),
@@ -219,7 +220,18 @@ TEST(ContextDeclsTest, InvalidTypeParamNameVariableValidationDisabled) {
               IsOk());
 }
 
-TEST(ContextDeclsTest, ErrorOnInvalidTypeParamNameVariable) {
+TEST(TypeCheckerBuilderImplTest, ErrorOnUnspecifiedMessageType) {
+  CheckerOptions options;
+  options.enable_type_parameter_name_validation = true;
+  TypeCheckerBuilderImpl builder(internal::GetSharedTestingDescriptorPool(),
+                                 options);
+  ASSERT_THAT(
+      builder.AddVariable(MakeVariableDecl("x", MessageType())),
+      StatusIs(absl::StatusCode::kInvalidArgument,
+               "an empty message type cannot be used in a type declaration"));
+}
+
+TEST(TypeCheckerBuilderImplTest, ErrorOnInvalidTypeParamNameVariable) {
   CheckerOptions options;
   options.enable_type_parameter_name_validation = true;
   TypeCheckerBuilderImpl builder(internal::GetSharedTestingDescriptorPool(),
@@ -234,7 +246,7 @@ TEST(ContextDeclsTest, ErrorOnInvalidTypeParamNameVariable) {
                "type parameter name 'T% foo' is not a valid identifier"));
 }
 
-TEST(ContextDeclsTest, ErrorOnTooDeepTypeNestingVariable) {
+TEST(TypeCheckerBuilderImplTest, ErrorOnTooDeepTypeNestingVariable) {
   CheckerOptions options;
   options.max_type_decl_nesting = 2;
   google::protobuf::Arena arena;
@@ -251,7 +263,7 @@ TEST(ContextDeclsTest, ErrorOnTooDeepTypeNestingVariable) {
                "type nesting limit of 2 exceeded"));
 }
 
-TEST(ContextDeclsTest, ErrorOnInvalidTypeParamNameFunction) {
+TEST(TypeCheckerBuilderImplTest, ErrorOnInvalidTypeParamNameFunction) {
   CheckerOptions options;
   options.enable_type_parameter_name_validation = true;
   TypeCheckerBuilderImpl builder(internal::GetSharedTestingDescriptorPool(),
@@ -269,7 +281,7 @@ TEST(ContextDeclsTest, ErrorOnInvalidTypeParamNameFunction) {
                        "type parameter name '' is not a valid identifier"));
 }
 
-TEST(ContextDeclsTest, ErrorOnTooDeepTypeNestingFunction) {
+TEST(TypeCheckerBuilderImplTest, ErrorOnTooDeepTypeNestingFunction) {
   CheckerOptions options;
   options.max_type_decl_nesting = 2;
   google::protobuf::Arena arena;
@@ -294,7 +306,7 @@ TEST(ContextDeclsTest, ErrorOnTooDeepTypeNestingFunction) {
                        "type nesting limit of 2 exceeded"));
 }
 
-TEST(ContextDeclsTest, ReplaceVariable) {
+TEST(TypeCheckerBuilderImplTest, ReplaceVariable) {
   TypeCheckerBuilderImpl builder(internal::GetSharedTestingDescriptorPool(),
                                  {});
   ASSERT_THAT(
diff --git a/common/types/message_type.h b/common/types/message_type.h
index 232258170..782af87aa 100644
--- a/common/types/message_type.h
+++ b/common/types/message_type.h
@@ -57,6 +57,10 @@ class MessageType final {
         << descriptor->full_name();
   }
 
+  // Constructs a `MessageType` in an empty state.
+  //
+  // Most operations on an empty `MessageType` result in undefined behavior. Use
+  // `operator bool` to test if a `MessageType` is empty.
   MessageType() = default;
   MessageType(const MessageType&) = default;
   MessageType(MessageType&&) = default;