diff -r b3b4bee161cf -r a98e926b2f5b mercurial/revlog.py
--- a/mercurial/revlog.py	Mon Jul 23 16:21:58 2018 +0200
+++ b/mercurial/revlog.py	Fri Jul 20 13:20:01 2018 +0200
@@ -2514,6 +2514,11 @@
             else:
                 deltachain = []
 
+            # search for the first non-snapshot revision
+            for idx, r in enumerate(deltachain):
+                if not self.issnapshot(r):
+                    break
+            deltachain = deltachain[idx:]
             chunks = _slicechunk(self, deltachain, deltainfo)
             all_span = [_segmentspan(self, revs, deltainfo) for revs in chunks]
             distance = max(all_span)