diff --git a/src/lib-fs/fs-api-private.h b/src/lib-fs/fs-api-private.h
index d471dc46f6..5e152988a8 100644
--- a/src/lib-fs/fs-api-private.h
+++ b/src/lib-fs/fs-api-private.h
@@ -115,6 +115,7 @@ struct fs_file {
 	struct timeval timing_start[FS_OP_COUNT];
 
 	unsigned int write_pending:1;
+	unsigned int writing_stream:1;
 	unsigned int metadata_changed:1;
 
 	unsigned int read_or_prefetch_counted:1;
diff --git a/src/lib-fs/fs-api.c b/src/lib-fs/fs-api.c
index 1a2a5e2a45..ed7e179c02 100644
--- a/src/lib-fs/fs-api.c
+++ b/src/lib-fs/fs-api.c
@@ -262,6 +262,9 @@ void fs_file_deinit(struct fs_file **_file)
 
 void fs_file_close(struct fs_file *file)
 {
+	i_assert(!file->writing_stream);
+	i_assert(file->output == NULL);
+
 	if (file->pending_read_input != NULL)
 		i_stream_unref(&file->pending_read_input);
 	if (file->seekable_input != NULL)
@@ -632,6 +635,10 @@ int fs_write(struct fs_file *file, const void *data, size_t size)
 
 struct ostream *fs_write_stream(struct fs_file *file)
 {
+	i_assert(!file->writing_stream);
+	i_assert(file->output == NULL);
+
+	file->writing_stream = TRUE;
 	file->fs->stats.write_count++;
 	T_BEGIN {
 		file->fs->v.write_stream(file);
@@ -645,6 +652,8 @@ static int fs_write_stream_finish_int(struct fs_file *file, bool success)
 {
 	int ret;
 
+	i_assert(file->writing_stream);
+
 	fs_file_timing_start(file, FS_OP_WRITE);
 	T_BEGIN {
 		ret = file->fs->v.write_stream_finish(file, success);
@@ -657,6 +666,8 @@ static int fs_write_stream_finish_int(struct fs_file *file, bool success)
 		   indicated a failure. */
 		i_assert(success);
 	}
+	if (ret != 0)
+		file->writing_stream = FALSE;
 	return ret;
 }
 
@@ -908,6 +919,8 @@ int fs_delete(struct fs_file *file)
 {
 	int ret;
 
+	i_assert(!file->writing_stream);
+
 	fs_file_timing_start(file, FS_OP_DELETE);
 	T_BEGIN {
 		ret = file->fs->v.delete_file(file);