diff --git a/google/internal/externalaccount/executablecredsource.go b/google/internal/externalaccount/executablecredsource.go
index 2576dfd..90e8117 100644
--- a/google/internal/externalaccount/executablecredsource.go
+++ b/google/internal/externalaccount/executablecredsource.go
@@ -63,8 +63,8 @@ type executableResponse struct {
 	ExpirationTime *int64  `json:"expiration_time"`
 	IdToken        *string `json:"id_token"`
 	SamlResponse   *string `json:"saml_response"`
-	Code           *string `json:"code"`
-	Message        *string `json:"message"`
+	Code           string  `json:"code"`
+	Message        string  `json:"message"`
 }
 
 func parseSubjectToken(response []byte) (string, error) {
@@ -82,20 +82,10 @@ func parseSubjectToken(response []byte) (string, error) {
 	}
 
 	if !*result.Success {
-		details := ""
-		if result.Code != nil {
-			details += fmt.Sprintf("(%v)", *result.Code)
+		if result.Code == "" || result.Message == "" {
+			return "", errors.New("oauth2/google: Response must include `error` and `message` fields when unsuccessful.")
 		}
-		if result.Message != nil {
-			if details != "" {
-				details += " "
-			}
-			details += *result.Message
-		}
-		if details == "" {
-			details = "Unknown Error"
-		}
-		return "", fmt.Errorf("oauth2/google: Executable returned unsuccessful response: %v.", details)
+		return "", fmt.Errorf("oauth2/google: Executable returned unsuccessful response: (%v) %v.", result.Code, result.Message)
 	}
 
 	if *result.Version > EXECUTABLE_SUPPORTED_MAX_VERSION {
diff --git a/google/internal/externalaccount/executablecredsource_test.go b/google/internal/externalaccount/executablecredsource_test.go
index 6f9afde..fec3720 100644
--- a/google/internal/externalaccount/executablecredsource_test.go
+++ b/google/internal/externalaccount/executablecredsource_test.go
@@ -410,8 +410,8 @@ func TestRetrieveExecutableSubjectTokenUnsuccessfulResponseWithFields(t *testing
 		return json.Marshal(executableResponse{
 			Success: Bool(false),
 			Version: Int(1),
-			Code:    String("404"),
-			Message: String("Token Not Found"),
+			Code:    "404",
+			Message: "Token Not Found",
 		})
 	}
 
@@ -459,7 +459,7 @@ func TestRetrieveExecutableSubjectTokenUnsuccessfulResponseWithCode(t *testing.T
 		return json.Marshal(executableResponse{
 			Success: Bool(false),
 			Version: Int(1),
-			Code:    String("404"),
+			Code:    "404",
 		})
 	}
 
@@ -472,7 +472,7 @@ func TestRetrieveExecutableSubjectTokenUnsuccessfulResponseWithCode(t *testing.T
 	if err == nil {
 		t.Fatalf("Expected error but found none")
 	}
-	if got, want := err.Error(), "oauth2/google: Executable returned unsuccessful response: (404)."; got != want {
+	if got, want := err.Error(), "oauth2/google: Response must include `error` and `message` fields when unsuccessful."; got != want {
 		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
 	}
 
@@ -507,7 +507,7 @@ func TestRetrieveExecutableSubjectTokenUnsuccessfulResponseWithMessage(t *testin
 		return json.Marshal(executableResponse{
 			Success: Bool(false),
 			Version: Int(1),
-			Message: String("Token Not Found"),
+			Message: "Token Not Found",
 		})
 	}
 
@@ -520,7 +520,7 @@ func TestRetrieveExecutableSubjectTokenUnsuccessfulResponseWithMessage(t *testin
 	if err == nil {
 		t.Fatalf("Expected error but found none")
 	}
-	if got, want := err.Error(), "oauth2/google: Executable returned unsuccessful response: Token Not Found."; got != want {
+	if got, want := err.Error(), "oauth2/google: Response must include `error` and `message` fields when unsuccessful."; got != want {
 		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
 	}
 
@@ -567,7 +567,7 @@ func TestRetrieveExecutableSubjectTokenUnsuccessfulResponseWithoutFields(t *test
 	if err == nil {
 		t.Fatalf("Expected error but found none")
 	}
-	if got, want := err.Error(), "oauth2/google: Executable returned unsuccessful response: Unknown Error."; got != want {
+	if got, want := err.Error(), "oauth2/google: Response must include `error` and `message` fields when unsuccessful."; got != want {
 		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
 	}