diff --git a/src/hprof/hprof.h b/src/hprof/hprof.h
index bcdbef8..e2f7ce1 100644
--- a/src/hprof/hprof.h
+++ b/src/hprof/hprof.h
@@ -196,7 +196,7 @@
   HprofClassObjectId LookupClassId(Class* c);
   HprofStringId LookupStringId(String* string);
   HprofStringId LookupStringId(const char* string);
-  HprofStringId LookupStringId(std::string string);
+  HprofStringId LookupStringId(const std::string& string);
   HprofStringId LookupClassNameId(Class* c);
   static HprofBasicType SignatureToBasicTypeAndSize(const char* sig, size_t* sizeOut);
   static HprofBasicType PrimitiveToBasicTypeAndSize(Primitive::Type prim, size_t* sizeOut);