diff --git a/common/rc b/common/rc
index 857308a8..fbb19c2c 100644
--- a/common/rc
+++ b/common/rc
@@ -1111,10 +1111,12 @@ _require_scratch_nocheck()
     esac
 
     # mounted?
-    if _mount | grep -q $SCRATCH_DEV
+    # Note that we use -F here so grep doesn't try to interpret an NFS over
+    # IPv6 server as a regular expression.
+    if _mount | grep -F -q $SCRATCH_DEV
     then
         # if it's mounted, make sure its on $SCRATCH_MNT
-        if ! _mount | grep $SCRATCH_DEV | grep -q $SCRATCH_MNT
+        if ! _mount | grep -F $SCRATCH_DEV | grep -q $SCRATCH_MNT
         then
             echo "\$SCRATCH_DEV is mounted but not on \$SCRATCH_MNT - aborting"
             exit 1