diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index 19ea4debd..9791b4fed 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -206,7 +206,7 @@ void FdSource::restart()
     if (!isSeekable)
         throw Error("can't seek to the start of a file");
     buffer.reset();
-    read = bufPosOut = bufPosOut = 0;
+    read = bufPosIn = bufPosOut = 0;
     int fd_ = fromDescriptorReadOnly(fd);
     if (lseek(fd_, 0, SEEK_SET) == -1)
         throw SysError("seeking to the start of a file");