diff --git a/src/input/ProxyInputStream.cxx b/src/input/ProxyInputStream.cxx
index efa648d5e..86d2e82aa 100644
--- a/src/input/ProxyInputStream.cxx
+++ b/src/input/ProxyInputStream.cxx
@@ -20,6 +20,9 @@
 #include "config.h"
 #include "ProxyInputStream.hxx"
 #include "tag/Tag.hxx"
+#include "thread/Cond.hxx"
+
+#include <stdexcept>
 
 ProxyInputStream::ProxyInputStream(InputStreamPtr _input) noexcept
 	:InputStream(_input->GetURI(), _input->mutex, _input->cond),
@@ -30,9 +33,24 @@ ProxyInputStream::ProxyInputStream(InputStreamPtr _input) noexcept
 
 ProxyInputStream::~ProxyInputStream() noexcept = default;
 
+void
+ProxyInputStream::SetInput(InputStreamPtr _input) noexcept
+{
+	assert(!input);
+	assert(_input);
+
+	input = std::move(_input);
+
+	/* this call wakes up client threads if the new input is
+	   ready */
+	CopyAttributes();
+}
+
 void
 ProxyInputStream::CopyAttributes()
 {
+	assert(input);
+
 	if (input->IsReady()) {
 		if (!IsReady()) {
 			if (input->HasMimeType())
@@ -53,12 +71,16 @@ ProxyInputStream::CopyAttributes()
 void
 ProxyInputStream::Check()
 {
-	input->Check();
+	if (input)
+		input->Check();
 }
 
 void
 ProxyInputStream::Update() noexcept
 {
+	if (!input)
+		return;
+
 	input->Update();
 	CopyAttributes();
 }
@@ -66,6 +88,9 @@ ProxyInputStream::Update() noexcept
 void
 ProxyInputStream::Seek(offset_type new_offset)
 {
+	while (!input)
+		cond.wait(mutex);
+
 	input->Seek(new_offset);
 	CopyAttributes();
 }
@@ -73,24 +98,30 @@ ProxyInputStream::Seek(offset_type new_offset)
 bool
 ProxyInputStream::IsEOF() noexcept
 {
-	return input->IsEOF();
+	return input && input->IsEOF();
 }
 
 std::unique_ptr<Tag>
 ProxyInputStream::ReadTag()
 {
+	if (!input)
+		return nullptr;
+
 	return input->ReadTag();
 }
 
 bool
 ProxyInputStream::IsAvailable() noexcept
 {
-	return input->IsAvailable();
+	return input && input->IsAvailable();
 }
 
 size_t
 ProxyInputStream::Read(void *ptr, size_t read_size)
 {
+	while (!input)
+		cond.wait(mutex);
+
 	size_t nbytes = input->Read(ptr, read_size);
 	CopyAttributes();
 	return nbytes;
diff --git a/src/input/ProxyInputStream.hxx b/src/input/ProxyInputStream.hxx
index 6f768ad47..e55330a64 100644
--- a/src/input/ProxyInputStream.hxx
+++ b/src/input/ProxyInputStream.hxx
@@ -29,15 +29,25 @@ struct Tag;
  * An #InputStream that forwards all methods call to another
  * #InputStream instance.  This can be used as a base class to
  * override selected methods.
+ *
+ * The inner #InputStream instance may be nullptr initially, to be set
+ * later.
  */
 class ProxyInputStream : public InputStream {
 protected:
 	InputStreamPtr input;
 
 public:
-	gcc_nonnull_all
 	explicit ProxyInputStream(InputStreamPtr _input) noexcept;
 
+	/**
+	 * Construct an instance without an #InputStream instance.
+	 * Once that instance becomes available, call SetInput().
+	 */
+	ProxyInputStream(const char *_uri,
+			 Mutex &_mutex, Cond &_cond) noexcept
+		:InputStream(_uri, _mutex, _cond) {}
+
 	virtual ~ProxyInputStream() noexcept;
 
 	ProxyInputStream(const ProxyInputStream &) = delete;
@@ -53,6 +63,14 @@ public:
 	size_t Read(void *ptr, size_t read_size) override;
 
 protected:
+	/**
+	 * If this instance was initialized without an input, this
+	 * method can set it.
+	 *
+	 * Caller must lock the mutex.
+	 */
+	void SetInput(InputStreamPtr _input) noexcept;
+
 	/**
 	 * Copy public attributes from the underlying input stream to the
 	 * "rewind" input stream.  This function is called when a method of