1 files changed, 3 insertions, 0 deletions
diff --git a/io.c b/io.c
index a01aca8f61..2f4d1db5f8 100644
--- a/io.c
+++ b/io.c
@@ -1695,6 +1695,9 @@ io_allocate_write_buffer(rb_io_t *fptr, int sync)
         fptr->wbuf.len = 0;
         fptr->wbuf.capa = IO_WBUF_CAPA_MIN;
         fptr->wbuf.ptr = ALLOC_N(char, fptr->wbuf.capa);
+    }
+
+    if (!fptr->write_lock) {
         fptr->write_lock = rb_mutex_new();
         rb_mutex_allow_trap(fptr->write_lock, 1);
     }