diff --git a/runtime/vm/compiler/frontend/bytecode_reader.cc b/runtime/vm/compiler/frontend/bytecode_reader.cc
index 0e9c427..587eb27 100644
--- a/runtime/vm/compiler/frontend/bytecode_reader.cc
+++ b/runtime/vm/compiler/frontend/bytecode_reader.cc
@@ -57,6 +57,8 @@
     return;
   }
 
+  ASSERT(Thread::Current()->IsMutatorThread());
+
   AlternativeReadingScope alt(&helper_->reader_, &H.metadata_payloads(),
                               md_offset);
 
@@ -766,6 +768,7 @@
   ASSERT(!FLAG_precompiled_mode);
   ASSERT(!function.HasBytecode());
   ASSERT(thread->sticky_error() == Error::null());
+  ASSERT(Thread::Current()->IsMutatorThread());
 
   LongJumpScope jump;
   if (setjmp(*jump.Set()) == 0) {
diff --git a/runtime/vm/compiler/jit/compiler.cc b/runtime/vm/compiler/jit/compiler.cc
index e90f251..83c5fe9 100644
--- a/runtime/vm/compiler/jit/compiler.cc
+++ b/runtime/vm/compiler/jit/compiler.cc
@@ -228,6 +228,7 @@
 // Compile a function. Should call only if the function has not been compiled.
 //   Arg0: function object.
 DEFINE_RUNTIME_ENTRY(CompileFunction, 1) {
+  ASSERT(thread->IsMutatorThread());
   const Function& function = Function::CheckedHandle(arguments.ArgAt(0));
   Object& result = Object::Handle(zone);
 
@@ -1329,6 +1330,7 @@
 
 RawError* Compiler::ReadAllBytecode(const Class& cls) {
   Thread* thread = Thread::Current();
+  ASSERT(thread->IsMutatorThread());
   Zone* zone = thread->zone();
   Error& error = Error::Handle(zone, cls.EnsureIsFinalized(thread));
   ASSERT(error.IsNull());
@@ -1370,6 +1372,7 @@
   LongJumpScope jump;
   if (setjmp(*jump.Set()) == 0) {
     Thread* const thread = Thread::Current();
+    ASSERT(thread->IsMutatorThread());
     NoOOBMessageScope no_msg_scope(thread);
     NoReloadScope no_reload_scope(thread->isolate(), thread);
     // Under lazy compilation initializer has not yet been created, so create
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index f6bf794..44f95e8 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -12505,6 +12505,7 @@
 #if !defined(DART_PRECOMPILED_RUNTIME)
 RawError* Library::ReadAllBytecode() {
   Thread* thread = Thread::Current();
+  ASSERT(thread->IsMutatorThread());
   Zone* zone = thread->zone();
   Error& error = Error::Handle(zone);
   const GrowableObjectArray& libs = GrowableObjectArray::Handle(