diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 07e1679da..b2b5afadd 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1037,7 +1037,7 @@ StorePath LocalStore::addToStore(const string & name, const Path & _srcPath,
     FileIngestionMethod method, HashType hashAlgo, PathFilter & filter, RepairFlag repair)
 {
     Path srcPath(absPath(_srcPath));
-    auto source = sinkToSource([&](Sink & sink, size_t & wanted) {
+    auto source = sinkToSource([&](Sink & sink) {
         if (method == FileIngestionMethod::Recursive)
             dumpPath(srcPath, sink, filter);
         else
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index 4c72dc9f2..00c945113 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -165,43 +165,35 @@ size_t StringSource::read(unsigned char * data, size_t len)
 #endif
 
 std::unique_ptr<Source> sinkToSource(
-    std::function<void(Sink &, size_t &)> fun,
+    std::function<void(Sink &)> fun,
     std::function<void()> eof)
 {
     struct SinkToSource : Source
     {
-        typedef boost::coroutines2::coroutine<std::basic_string<uint8_t>> coro_t;
+        typedef boost::coroutines2::coroutine<std::string> coro_t;
 
-        std::function<void(Sink &, size_t &)> fun;
+        std::function<void(Sink &)> fun;
         std::function<void()> eof;
         std::optional<coro_t::pull_type> coro;
         bool started = false;
 
-        /* It would be nicer to have the co-routines have both args and a
-           return value, but unfortunately that was removed from Boost's
-           implementation for some reason, so we use some extra state instead.
-           */
-       size_t wanted = 0;
-
-        SinkToSource(std::function<void(Sink &, size_t &)> fun, std::function<void()> eof)
+        SinkToSource(std::function<void(Sink &)> fun, std::function<void()> eof)
             : fun(fun), eof(eof)
         {
         }
 
-        std::basic_string<uint8_t> cur;
+        std::string cur;
         size_t pos = 0;
 
         size_t read(unsigned char * data, size_t len) override
         {
-            wanted = len < cur.size() ? 0 : len - cur.size();
-            if (!coro) {
+            if (!coro)
                 coro = coro_t::pull_type([&](coro_t::push_type & yield) {
-                    LambdaSink sink([&](const uint8_t * data, size_t len) {
-                            if (len) yield(std::basic_string<uint8_t> { data, len });
+                    LambdaSink sink([&](const unsigned char * data, size_t len) {
+                            if (len) yield(std::string((const char *) data, len));
                         });
-                    fun(sink, wanted);
+                    fun(sink);
                 });
-            }
 
             if (!*coro) { eof(); abort(); }
 
@@ -211,10 +203,11 @@ std::unique_ptr<Source> sinkToSource(
                 pos = 0;
             }
 
-            auto numCopied = cur.copy(data, len, pos);
-            pos += numCopied;
+            auto n = std::min(cur.size() - pos, len);
+            memcpy(data, (unsigned char *) cur.data() + pos, n);
+            pos += n;
 
-            return numCopied;
+            return n;
         }
     };
 
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 3e3735ca5..aa6b42597 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -273,19 +273,10 @@ struct ChainSource : Source
 /* Convert a function that feeds data into a Sink into a Source. The
    Source executes the function as a coroutine. */
 std::unique_ptr<Source> sinkToSource(
-    std::function<void(Sink &, size_t &)> fun,
-    std::function<void()> eof = []() {
-        throw EndOfFile("coroutine has finished");
-    });
-
-static inline std::unique_ptr<Source> sinkToSource(
     std::function<void(Sink &)> fun,
     std::function<void()> eof = []() {
         throw EndOfFile("coroutine has finished");
-    })
-{
-    return sinkToSource([fun](Sink & s, size_t & _) { fun(s); }, eof);
-}
+    });
 
 
 void writePadding(size_t len, Sink & sink);