diff --git a/coregrind/m_libcprint.c b/coregrind/m_libcprint.c
index e425590..a9cc906 100644
--- a/coregrind/m_libcprint.c
+++ b/coregrind/m_libcprint.c
@@ -169,6 +169,8 @@
       b->buf[b->buf_used++] = c;
       if (b->buf_used < b->buf_size)
          b->buf[b->buf_used] = 0;
+      else
+         b->buf[b->buf_size-1] = 0; /* pre: b->buf_size > 0 */
    } 
 }