2 files changed, 5 insertions, 3 deletions
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index a3cd4ff0d02c..5c45c890f7b6 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -288,11 +288,11 @@ template PathSet readStrings(Source & source);
 void StringSink::operator () (const unsigned char * data, size_t len)
 {
     static bool warned = false;
-    if (!warned && s.size() > threshold) {
+    if (!warned && s->size() > threshold) {
         warnLargeDump();
         warned = true;
     }
-    s.append((const char *) data, len);
+    s->append((const char *) data, len);
 }
 
 
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 0fdc4037b2d7..9ba6391f817a 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -110,7 +110,9 @@ private:
 /* A sink that writes data to a string. */
 struct StringSink : Sink
 {
-    string s;
+    ref<std::string> s;
+    StringSink() : s(make_ref<std::string>()) { };
+    StringSink(ref<std::string> s) : s(s) { };
     void operator () (const unsigned char * data, size_t len) override;
 };