Hide namespace alias protobuf into protobuf_mutator namespace.
diff --git a/src/protobuf_mutator.cc b/src/protobuf_mutator.cc
index b69beab..eaebbfb 100644
--- a/src/protobuf_mutator.cc
+++ b/src/protobuf_mutator.cc
@@ -23,6 +23,8 @@
 #include "src/field_instance.h"
 #include "src/weighted_reservoir_sampler.h"
 
+namespace protobuf_mutator {
+
 using protobuf::Descriptor;
 using protobuf::EnumDescriptor;
 using protobuf::EnumValueDescriptor;
@@ -31,8 +33,6 @@
 using protobuf::OneofDescriptor;
 using protobuf::Reflection;
 
-namespace protobuf_mutator {
-
 namespace {
 
 const size_t kMaxInitializeDepth = 32;