diff --git a/pkg/_fe_analyzer_shared/lib/src/macros/executor/serialization_extensions.dart b/pkg/_fe_analyzer_shared/lib/src/macros/executor/serialization_extensions.dart
index c2fe1bc5c2c..43bd1239363 100644
--- a/pkg/_fe_analyzer_shared/lib/src/macros/executor/serialization_extensions.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/macros/executor/serialization_extensions.dart
@@ -501,9 +501,11 @@ extension DeserializerExtensions on Deserializer {
         new DiagnosticMessage(message, target: target.asDiagnosticTarget),
       TypeAnnotationImpl() =>
         new DiagnosticMessage(message, target: target.asDiagnosticTarget),
+      MetadataAnnotationImpl() =>
+        new DiagnosticMessage(message, target: target.asDiagnosticTarget),
       _ => throw new UnsupportedError(
-          'Unsupported target type ${target.runtimeType}, only Declarations '
-          'and TypeAnnotations are allowed.'),
+          'Unsupported target type ${target.runtimeType}, only Declarations, '
+          'TypeAnnotations, and Metadata are allowed.'),
     };
   }
 }
diff --git a/tests/language/macros/json/json_serializable_diagnostics_test.dart b/tests/language/macros/json/json_serializable_diagnostics_test.dart
index 2c3972a9f02..c00dd58e678 100644
--- a/tests/language/macros/json/json_serializable_diagnostics_test.dart
+++ b/tests/language/macros/json/json_serializable_diagnostics_test.dart
@@ -85,7 +85,7 @@ class EnumField {
 }
 
 @JsonSerializable()
-class EnumField {
+class DuplicateJsonKey {
   @JsonKey()
   @JsonKey()
 //^^^^^^^^^^