diff --git a/src/lib/iostream-temp.c b/src/lib/iostream-temp.c
index 9660361133..f3776c6af8 100644
--- a/src/lib/iostream-temp.c
+++ b/src/lib/iostream-temp.c
@@ -131,8 +131,11 @@ static int o_stream_temp_dup_cancel(struct temp_ostream *tstream)
 	int ret = -1;
 
 	i_stream_seek(tstream->dupstream, tstream->dupstream_start_offset);
+	tstream->ostream.ostream.offset = 0;
 
 	input = i_stream_create_limit(tstream->dupstream, size);
+	i_stream_unref(&tstream->dupstream);
+
 	if (io_stream_copy(&tstream->ostream.ostream, input) > 0) {
 		/* everything copied */
 		ret = 0;
@@ -141,7 +144,6 @@ static int o_stream_temp_dup_cancel(struct temp_ostream *tstream)
 		tstream->ostream.ostream.stream_errno = input->stream_errno;
 	}
 	i_stream_destroy(&input);
-	i_stream_unref(&tstream->dupstream);
 	return ret;
 }
 
@@ -170,7 +172,12 @@ static int o_stream_temp_dup_istream(struct temp_ostream *outstream,
 			return o_stream_temp_dup_cancel(outstream);
 	}
 	i_stream_seek(instream, in_size);
+	/* we should be at EOF now. o_stream_send_istream() asserts if
+	   eof isn't set. */
+	instream->eof = TRUE;
 	outstream->dupstream_offset = instream->v_offset;
+	outstream->ostream.ostream.offset =
+		outstream->dupstream_offset - outstream->dupstream_start_offset;
 	return 1;
 }
 
diff --git a/src/lib/test-iostream-temp.c b/src/lib/test-iostream-temp.c
index 7cfae398c3..d2978a6e24 100644
--- a/src/lib/test-iostream-temp.c
+++ b/src/lib/test-iostream-temp.c
@@ -1,9 +1,13 @@
 /* Copyright (c) 2016 Dovecot authors, see the included COPYING file */
 
 #include "test-lib.h"
+#include "istream.h"
 #include "ostream.h"
 #include "iostream-temp.h"
 
+#include <unistd.h>
+#include <fcntl.h>
+
 static void test_iostream_temp_create_sized_memory(void)
 {
 	struct ostream *output;
@@ -39,8 +43,60 @@ static void test_iostream_temp_create_sized_disk(void)
 	test_end();
 }
 
+static void test_iostream_temp_istream(void)
+{
+	struct istream *input, *input2, *temp_input;
+	struct ostream *output;
+	int fd;
+
+	test_begin("iostream_temp istream");
+
+	fd = open(".temp.istream", O_RDWR | O_CREAT | O_TRUNC, 0600);
+	if (fd == -1)
+		i_fatal("create(.temp.istream) failed: %m");
+	test_assert(write(fd, "foobar", 6) == 6);
+	test_assert(lseek(fd, 0, SEEK_SET) == 0);
+
+	input = i_stream_create_fd(fd, 1024, TRUE);
+	/* a working fd-dup */
+	output = iostream_temp_create_sized(".nonexistent/",
+		IOSTREAM_TEMP_FLAG_TRY_FD_DUP, "test", 1);
+	test_assert(o_stream_send_istream(output, input) > 0);
+	temp_input = iostream_temp_finish(&output, 128);
+	test_assert(i_stream_read(temp_input) == 6);
+	i_stream_destroy(&temp_input);
+
+	/* non-working fd-dup: write data before sending istream */
+	i_stream_seek(input, 0);
+	output = iostream_temp_create_sized(".intentional-nonexistent-error/",
+		IOSTREAM_TEMP_FLAG_TRY_FD_DUP, "test", 4);
+	test_assert(o_stream_send(output, "1234", 4) == 4);
+	test_expect_errors(1);
+	test_assert(o_stream_send_istream(output, input) > 0);
+	test_expect_no_more_errors();
+	o_stream_destroy(&output);
+
+	/* non-working fd-dup: send two istreams */
+	i_stream_seek(input, 0);
+	input2 = i_stream_create_limit(input, (uoff_t)-1);
+	output = iostream_temp_create_sized(".intentional-nonexistent-error/",
+		IOSTREAM_TEMP_FLAG_TRY_FD_DUP, "test", 4);
+	test_assert(o_stream_send_istream(output, input) > 0);
+	test_expect_errors(1);
+	test_assert(o_stream_send_istream(output, input2) > 0);
+	test_expect_no_more_errors();
+	o_stream_destroy(&output);
+	i_stream_unref(&input2);
+
+	i_stream_destroy(&input);
+
+	i_unlink(".temp.istream");
+	test_end();
+}
+
 void test_iostream_temp(void)
 {
 	test_iostream_temp_create_sized_memory();
 	test_iostream_temp_create_sized_disk();
+	test_iostream_temp_istream();
 }