--- a/mercurial/changegroup.py	Tue Feb 06 16:08:57 2018 -0800
+++ b/mercurial/changegroup.py	Tue Feb 06 19:08:25 2018 -0800
@@ -770,6 +770,8 @@
         progress(msgbundling, None)
 
     def deltaparent(self, revlog, rev, p1, p2, prev):
+        if not revlog.candelta(prev, rev):
+            raise error.ProgrammingError('cg1 should not be used in this case')
         return prev
 
     def revchunk(self, revlog, rev, prev, linknode):
@@ -829,16 +831,19 @@
             # expensive. The revlog caches should have prev cached, meaning
             # less CPU for changegroup generation. There is likely room to add
             # a flag and/or config option to control this behavior.
-            return prev
+            base = prev
         elif dp == nullrev:
             # revlog is configured to use full snapshot for a reason,
             # stick to full snapshot.
-            return nullrev
+            base = nullrev
         elif dp not in (p1, p2, prev):
             # Pick prev when we can't be sure remote has the base revision.
             return prev
         else:
-            return dp
+            base = dp
+        if base != nullrev and not revlog.candelta(base, rev):
+            base = nullrev
+        return base
 
     def builddeltaheader(self, node, p1n, p2n, basenode, linknode, flags):
         # Do nothing with flags, it is implicitly 0 in cg1 and cg2