diff --git a/objectivec/GPBDescriptor.m b/objectivec/GPBDescriptor.m
index 6f2fa9b590f0..5f769d420a75 100644
--- a/objectivec/GPBDescriptor.m
+++ b/objectivec/GPBDescriptor.m
@@ -267,6 +267,10 @@ - (instancetype)initWithClass:(Class)messageClass
                        fields:(NSArray *)fields
                   storageSize:(uint32_t)storageSize
                    wireFormat:(BOOL)wireFormat {
+#if defined(DEBUG) && DEBUG && !defined(NS_BLOCK_ASSERTIONS)
+  // This is also checked by the generator.
+  NSAssert(!wireFormat || fields.count == 0, @"Internal error: MessageSets should not have fields");
+#endif
   if ((self = [super init])) {
     messageClass_ = messageClass;
     messageName_ = [messageName copy];
@@ -1139,8 +1143,16 @@ - (instancetype)initWithExtensionDescription:(GPBExtensionDescription *)desc
     GPBRuntimeMatchFailure();
   }
 
-#if defined(DEBUG) && DEBUG
+#if defined(DEBUG) && DEBUG && !defined(NS_BLOCK_ASSERTIONS)
   NSAssert(usesClassRefs, @"Internal error: all extensions should have class refs");
+
+  // This is also checked by the generator.
+  // If the extension is a MessageSet extension, then it must be a message field.
+  NSAssert(
+      ((desc->options & GPBExtensionSetWireFormat) == 0) || desc->dataType == GPBDataTypeMessage,
+      @"Internal error: If a MessageSet extension is set, the data type must be a message.");
+  // NOTE: Could also check that the exteneded class is a MessageSet, but that would force the ObjC
+  // runtime to start up that class and that isn't desirable here.
 #endif
 
   if ((self = [super init])) {
diff --git a/src/google/protobuf/compiler/objectivec/extension.cc b/src/google/protobuf/compiler/objectivec/extension.cc
index 44a750593425..528efa154ae8 100644
--- a/src/google/protobuf/compiler/objectivec/extension.cc
+++ b/src/google/protobuf/compiler/objectivec/extension.cc
@@ -39,6 +39,11 @@ ExtensionGenerator::ExtensionGenerator(
   ABSL_CHECK(!descriptor->is_map())
       << "error: Extension is a map<>!"
       << " That used to be blocked by the compiler.";
+  ABSL_CHECK(
+      !descriptor->containing_type()->options().message_set_wire_format() ||
+      descriptor->type() == FieldDescriptor::TYPE_MESSAGE)
+      << "error: Extension to a message_set_wire_format message and the type "
+         "wasn't a message!";
 }
 
 void ExtensionGenerator::GenerateMembersHeader(io::Printer* printer) const {
diff --git a/src/google/protobuf/compiler/objectivec/message.cc b/src/google/protobuf/compiler/objectivec/message.cc
index cde92f60d008..984376b24955 100644
--- a/src/google/protobuf/compiler/objectivec/message.cc
+++ b/src/google/protobuf/compiler/objectivec/message.cc
@@ -133,7 +133,7 @@ struct ExtensionRangeOrdering {
 
 // This is a reduced case of Descriptor::ExtensionRange with just start and end.
 struct SimpleExtensionRange {
-  SimpleExtensionRange(int start, int end) : start(start), end(end){};
+  SimpleExtensionRange(int start, int end) : start(start), end(end) {};
   int start;  // inclusive
   int end;    // exclusive
 
@@ -202,8 +202,12 @@ MessageGenerator::MessageGenerator(const std::string& file_description_name,
       class_name_(ClassName(descriptor_)),
       deprecated_attribute_(
           GetOptionalDeprecatedAttribute(descriptor, descriptor->file())) {
-  ABSL_DCHECK(!descriptor->options().map_entry())
+  ABSL_CHECK(!descriptor->options().map_entry())
       << "error: MessageGenerator create of a map<>!";
+  ABSL_CHECK(!descriptor->options().message_set_wire_format() ||
+             descriptor->field_count() == 0)
+      << "error: MessageGenerator message_set_wire_format should never have "
+         "fields!";
   for (int i = 0; i < descriptor_->real_oneof_decl_count(); i++) {
     oneof_generators_.push_back(std::make_unique<OneofGenerator>(
         descriptor_->real_oneof_decl(i), generation_options));