diff --git a/base/BUILD b/base/BUILD
index 5820966d1..a239d4751 100644
--- a/base/BUILD
+++ b/base/BUILD
@@ -35,6 +35,7 @@ cc_library(
         ":kind",
         "//internal:status_macros",
         "@com_google_absl//absl/base:core_headers",
+        "@com_google_absl//absl/base:nullability",
         "@com_google_absl//absl/container:btree",
         "@com_google_absl//absl/status",
         "@com_google_absl//absl/status:statusor",
diff --git a/base/attribute.cc b/base/attribute.cc
index bf0f0c10d..f750a1850 100644
--- a/base/attribute.cc
+++ b/base/attribute.cc
@@ -19,6 +19,7 @@
 #include <string>
 
 #include "absl/base/macros.h"
+#include "absl/base/nullability.h"
 #include "absl/status/status.h"
 #include "absl/status/statusor.h"
 #include "absl/strings/str_cat.h"
@@ -70,6 +71,46 @@ class AttributeStringPrinter {
   Kind type_;
 };
 
+// Visitor for appending string representation for different qualifier kinds.
+class AttributeQualifierStringPrinter {
+ public:
+  // String representation for the given qualifier is appended to output.
+  explicit AttributeQualifierStringPrinter(std::string* absl_nonnull output,
+                                           Kind type)
+      : output_(*output), type_(type) {}
+
+  absl::Status operator()(const Kind& ignored) const {
+    // Attributes are represented as a variant, with illegal attribute
+    // qualifiers represented with their type as the first alternative.
+    return absl::InvalidArgumentError(
+        absl::StrCat("Unsupported attribute qualifier ", KindToString(type_)));
+  }
+
+  absl::Status operator()(int64_t index) {
+    absl::StrAppend(&output_, index);
+    return absl::OkStatus();
+  }
+
+  absl::Status operator()(uint64_t index) {
+    absl::StrAppend(&output_, index);
+    return absl::OkStatus();
+  }
+
+  absl::Status operator()(bool bool_key) {
+    absl::StrAppend(&output_, (bool_key) ? "true" : "false");
+    return absl::OkStatus();
+  }
+
+  absl::Status operator()(const std::string& field) {
+    absl::StrAppend(&output_, field);
+    return absl::OkStatus();
+  }
+
+ private:
+  std::string& output_;
+  Kind type_;
+};
+
 struct AttributeQualifierTypeVisitor final {
   Kind operator()(const Kind& type) const { return type; }
 
@@ -279,4 +320,11 @@ bool AttributeQualifier::IsMatch(const AttributeQualifier& other) const {
   return value_ == other.value_;
 }
 
+absl::StatusOr<std::string> AttributeQualifier::AsString() const {
+  std::string result;
+  CEL_RETURN_IF_ERROR(
+      absl::visit(AttributeQualifierStringPrinter(&result, kind()), value_));
+  return result;
+}
+
 }  // namespace cel
diff --git a/base/attribute.h b/base/attribute.h
index 91dc98700..69dcaf161 100644
--- a/base/attribute.h
+++ b/base/attribute.h
@@ -105,6 +105,8 @@ class AttributeQualifier final {
     return (key.has_value() && key.value() == other_key);
   }
 
+  absl::StatusOr<std::string> AsString() const;
+
  private:
   friend class Attribute;
   friend struct ComparatorVisitor;
diff --git a/eval/public/BUILD b/eval/public/BUILD
index f916f16d6..4e25c0481 100644
--- a/eval/public/BUILD
+++ b/eval/public/BUILD
@@ -681,10 +681,12 @@ cc_test(
         ":cel_attribute",
         ":cel_value",
         "//eval/public/structs:cel_proto_wrapper",
-        "//internal:status_macros",
         "//internal:testing",
         "@com_google_absl//absl/status",
+        "@com_google_absl//absl/status:status_matchers",
+        "@com_google_absl//absl/status:statusor",
         "@com_google_absl//absl/strings",
+        "@com_google_absl//absl/types:optional",
         "@com_google_protobuf//:protobuf",
     ],
 )
diff --git a/eval/public/cel_attribute_test.cc b/eval/public/cel_attribute_test.cc
index f595ae97d..b72189332 100644
--- a/eval/public/cel_attribute_test.cc
+++ b/eval/public/cel_attribute_test.cc
@@ -3,10 +3,12 @@
 #include <string>
 
 #include "absl/status/status.h"
+#include "absl/status/status_matchers.h"
+#include "absl/status/statusor.h"
 #include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
 #include "eval/public/cel_value.h"
 #include "eval/public/structs/cel_proto_wrapper.h"
-#include "internal/status_macros.h"
 #include "internal/testing.h"
 #include "google/protobuf/arena.h"
 
@@ -15,9 +17,8 @@ namespace {
 
 using cel::expr::Expr;
 
+using ::absl_testing::IsOkAndHolds;
 using ::absl_testing::StatusIs;
-using ::google::protobuf::Duration;
-using ::google::protobuf::Timestamp;
 using ::testing::Eq;
 using ::testing::IsEmpty;
 using ::testing::SizeIs;
@@ -51,17 +52,19 @@ TEST(CelAttributeQualifierTest, TestBoolAccess) {
   EXPECT_FALSE(qualifier.GetUint64Key().has_value());
   EXPECT_TRUE(qualifier.GetBoolKey().has_value());
   EXPECT_THAT(qualifier.GetBoolKey().value(), Eq(true));
+  EXPECT_THAT(qualifier.AsString(), IsOkAndHolds("true"));
 }
 
 TEST(CelAttributeQualifierTest, TestInt64Access) {
-  auto qualifier = CreateCelAttributeQualifier(CelValue::CreateInt64(1));
+  auto qualifier = CreateCelAttributeQualifier(CelValue::CreateInt64(-1));
 
   EXPECT_FALSE(qualifier.GetBoolKey().has_value());
   EXPECT_FALSE(qualifier.GetStringKey().has_value());
   EXPECT_FALSE(qualifier.GetUint64Key().has_value());
 
   EXPECT_TRUE(qualifier.GetInt64Key().has_value());
-  EXPECT_THAT(qualifier.GetInt64Key().value(), Eq(1));
+  EXPECT_THAT(qualifier.GetInt64Key().value(), Eq(-1));
+  EXPECT_THAT(qualifier.AsString(), IsOkAndHolds("-1"));
 }
 
 TEST(CelAttributeQualifierTest, TestUint64Access) {
@@ -73,6 +76,7 @@ TEST(CelAttributeQualifierTest, TestUint64Access) {
 
   EXPECT_TRUE(qualifier.GetUint64Key().has_value());
   EXPECT_THAT(qualifier.GetUint64Key().value(), Eq(1UL));
+  EXPECT_THAT(qualifier.AsString(), IsOkAndHolds("1"));
 }
 
 TEST(CelAttributeQualifierTest, TestStringAccess) {
@@ -85,6 +89,7 @@ TEST(CelAttributeQualifierTest, TestStringAccess) {
 
   EXPECT_TRUE(qualifier.GetStringKey().has_value());
   EXPECT_THAT(qualifier.GetStringKey().value(), Eq("test"));
+  EXPECT_THAT(qualifier.AsString(), IsOkAndHolds("test"));
 }
 
 void TestAllInequalities(const CelAttributeQualifier& qualifier) {
diff --git a/extensions/protobuf/internal/qualify.cc b/extensions/protobuf/internal/qualify.cc
index 411244744..6eac7e1d8 100644
--- a/extensions/protobuf/internal/qualify.cc
+++ b/extensions/protobuf/internal/qualify.cc
@@ -337,7 +337,12 @@ absl::StatusOr<google::protobuf::MapValueConstRef> ProtoQualifyState::CheckMapIn
                      qualifier));
 
   if (!value_ref.has_value()) {
-    return runtime_internal::CreateNoSuchKeyError("");
+    std::string key_string;
+    absl::StatusOr<std::string> key_string_or = qualifier.AsString();
+    if (key_string_or.ok()) {
+      key_string = *key_string_or;
+    }
+    return runtime_internal::CreateNoSuchKeyError(key_string);
   }
   return std::move(value_ref).value();
 }
diff --git a/extensions/select_optimization_test.cc b/extensions/select_optimization_test.cc
index 7a861bd13..24c86c560 100644
--- a/extensions/select_optimization_test.cc
+++ b/extensions/select_optimization_test.cc
@@ -101,6 +101,7 @@ using ::google::api::expr::runtime::LegacyTypeInfoApis;
 using ::google::api::expr::runtime::LegacyTypeMutationApis;
 using ::google::protobuf::Empty;
 using ::testing::_;
+using ::testing::AllOf;
 using ::testing::AnyOf;
 using ::testing::ElementsAre;
 using ::testing::Eq;
@@ -1222,7 +1223,8 @@ INSTANTIATE_TEST_SUITE_P(
               ASSERT_TRUE(result->Is<ErrorValue>()) << result->DebugString();
               EXPECT_THAT(result.GetError().NativeValue(),
                           StatusIs(absl::StatusCode::kNotFound,
-                                   HasSubstr("Key not found")));
+                                   AllOf(HasSubstr("Key not found"),
+                                         HasSubstr("$not_a_field"))));
             },
         },
         {