diff --git a/runtime/jni_env_ext.cc b/runtime/jni_env_ext.cc
index b2d3835..84fc404 100644
--- a/runtime/jni_env_ext.cc
+++ b/runtime/jni_env_ext.cc
@@ -28,11 +28,29 @@
 
 static constexpr size_t kLocalsInitial = 64;  // Arbitrary.
 
+// Checking "locals" requires the mutator lock, but at creation time we're really only interested
+// in validity, which isn't changing. To avoid grabbing the mutator lock, factored out and tagged
+// with NO_THREAD_SAFETY_ANALYSIS.
+static bool CheckLocalsValid(JNIEnvExt* in) NO_THREAD_SAFETY_ANALYSIS {
+  if (in == nullptr) {
+    return false;
+  }
+  return in->locals.IsValid();
+}
+
+JNIEnvExt* JNIEnvExt::Create(Thread* self_in, JavaVMExt* vm_in) {
+  std::unique_ptr<JNIEnvExt> ret(new JNIEnvExt(self_in, vm_in));
+  if (CheckLocalsValid(ret.get())) {
+    return ret.release();
+  }
+  return nullptr;
+}
+
 JNIEnvExt::JNIEnvExt(Thread* self_in, JavaVMExt* vm_in)
     : self(self_in),
       vm(vm_in),
       local_ref_cookie(IRT_FIRST_SEGMENT),
-      locals(kLocalsInitial, kLocalsMax, kLocal),
+      locals(kLocalsInitial, kLocalsMax, kLocal, false),
       check_jni(false),
       critical(0),
       monitors("monitors", kMonitorsInitial, kMonitorsMax) {