diff --git a/internal/impl/codec_extension.go b/internal/impl/codec_extension.go
index 141a3cd..1098159 100644
--- a/internal/impl/codec_extension.go
+++ b/internal/impl/codec_extension.go
@@ -110,6 +110,9 @@
 func (f *ExtensionField) lazyInit() {
 	f.lazy.mu.Lock()
 	defer f.lazy.mu.Unlock()
+	if atomic.LoadUint32(&f.lazy.atomicOnce) == 1 {
+		return
+	}
 	if f.lazy.xi != nil {
 		b := f.lazy.b
 		val := f.typ.New()
@@ -133,7 +136,7 @@
 			wtyp := wire.Type(tag & 7)
 			var out unmarshalOutput
 			var err error
-			val, out, err = f.lazy.xi.funcs.unmarshal(b, val, num, wtyp, unmarshalOptions{}) // TODO: options
+			val, out, err = f.lazy.xi.funcs.unmarshal(b, val, num, wtyp, unmarshalOptions{})
 			if err != nil {
 				panic(errors.New("decode failure in lazy extension decoding: %v", err))
 			}
diff --git a/proto/extension_test.go b/proto/extension_test.go
index cf1ef6c..b3a0cff 100644
--- a/proto/extension_test.go
+++ b/proto/extension_test.go
@@ -6,6 +6,7 @@
 
 import (
 	"fmt"
+	"sync"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -66,3 +67,39 @@
 
 	}
 }
+
+func TestExtensionGetRace(t *testing.T) {
+	// Concurrently fetch an extension value while marshaling the message containing it.
+	// Create the message with proto.Unmarshal to give lazy extension decoding (if present)
+	// a chance to occur.
+	want := int32(42)
+	m1 := &testpb.TestAllExtensions{}
+	proto.SetExtension(m1, testpb.E_OptionalNestedMessageExtension, &testpb.TestAllExtensions_NestedMessage{A: proto.Int32(want)})
+	b, err := proto.Marshal(m1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	m := &testpb.TestAllExtensions{}
+	if err := proto.Unmarshal(b, m); err != nil {
+		t.Fatal(err)
+	}
+	var wg sync.WaitGroup
+	for i := 0; i < 3; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if _, err := proto.Marshal(m); err != nil {
+				t.Error(err)
+			}
+		}()
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			got := proto.GetExtension(m, testpb.E_OptionalNestedMessageExtension).(*testpb.TestAllExtensions_NestedMessage).GetA()
+			if got != want {
+				t.Errorf("GetExtension(optional_nested_message).a = %v, want %v", got, want)
+			}
+		}()
+	}
+	wg.Wait()
+}