diff --git a/src/hotspot/share/cds/dynamicArchive.cpp b/src/hotspot/share/cds/dynamicArchive.cpp
index 065597c652d..47c3642ff0e 100644
--- a/src/hotspot/share/cds/dynamicArchive.cpp
+++ b/src/hotspot/share/cds/dynamicArchive.cpp
@@ -107,9 +107,11 @@ class DynamicArchiveBuilder : public ArchiveBuilder {
 
     verify_universe("Before CDS dynamic dump");
     DEBUG_ONLY(SystemDictionaryShared::NoClassLoadingMark nclm);
-    SystemDictionaryShared::check_excluded_classes();
 
+    // Block concurrent class unloading from changing the _dumptime_table
     MutexLocker ml(DumpTimeTable_lock, Mutex::_no_safepoint_check_flag);
+    SystemDictionaryShared::check_excluded_classes();
+
     init_header();
     gather_source_objs();
     reserve_buffer();
diff --git a/src/hotspot/share/cds/metaspaceShared.cpp b/src/hotspot/share/cds/metaspaceShared.cpp
index 0ea1abb2abf..c8ec44aee06 100644
--- a/src/hotspot/share/cds/metaspaceShared.cpp
+++ b/src/hotspot/share/cds/metaspaceShared.cpp
@@ -484,12 +484,10 @@ void VM_PopulateDumpSharedSpace::doit() {
 
   NOT_PRODUCT(SystemDictionary::verify();)
 
-  // At this point, many classes have been loaded.
-  // Gather systemDictionary classes in a global array and do everything to
-  // that so we don't have to walk the SystemDictionary again.
+  // Block concurrent class unloading from changing the _dumptime_table
+  MutexLocker ml(DumpTimeTable_lock, Mutex::_no_safepoint_check_flag);
   SystemDictionaryShared::check_excluded_classes();
 
-  MutexLocker ml(DumpTimeTable_lock, Mutex::_no_safepoint_check_flag);
   StaticArchiveBuilder builder;
   builder.gather_source_objs();
   builder.reserve_buffer();
diff --git a/src/hotspot/share/classfile/systemDictionaryShared.cpp b/src/hotspot/share/classfile/systemDictionaryShared.cpp
index 811feb30f54..78fcb1bc8e2 100644
--- a/src/hotspot/share/classfile/systemDictionaryShared.cpp
+++ b/src/hotspot/share/classfile/systemDictionaryShared.cpp
@@ -1485,6 +1485,8 @@ class ExcludeDumpTimeSharedClasses : StackObj {
 };
 
 void SystemDictionaryShared::check_excluded_classes() {
+  assert(no_class_loading_should_happen(), "sanity");
+  assert_lock_strong(DumpTimeTable_lock);
   ExcludeDumpTimeSharedClasses excl;
   _dumptime_table->iterate(&excl);
   _dumptime_table->update_counts();