diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -497,6 +497,9 @@ class revlog(object):
         else:
             raise error.RevlogError(_('unknown version (%d) in revlog %s') %
                                     (fmt, self.indexfile))
+        # sparse-revlog can't be on without general-delta (issue6056)
+        if not self._generaldelta:
+            self._sparserevlog = False
 
         self._storedeltachains = True
 
diff --git a/mercurial/revlogutils/deltas.py b/mercurial/revlogutils/deltas.py
--- a/mercurial/revlogutils/deltas.py
+++ b/mercurial/revlogutils/deltas.py
@@ -755,7 +755,8 @@ def _rawgroups(revlog, p1, p2, cachedelt
     The group order aims at providing fast or small candidates first.
     """
     gdelta = revlog._generaldelta
-    sparse = revlog._sparserevlog
+    # gate sparse behind general-delta because of issue6056
+    sparse = gdelta and revlog._sparserevlog
     curr = len(revlog)
     prev = curr - 1
     deltachain = lambda rev: revlog._deltachain(rev)[0]
diff --git a/tests/test-init.t b/tests/test-init.t
--- a/tests/test-init.t
+++ b/tests/test-init.t
@@ -268,3 +268,25 @@ clone bookmarks
   exporting bookmark test
   $ hg -R remote-bookmarks bookmarks
      test                      0:08b9e9f63b32
+
+Check format constraint
+-----------------------
+
+  $ hg init issue6056 --config format.usegeneraldelta=0 --config format.sparse-revlog=0
+  $ cd issue6056
+  $ echo foo > 1
+  $ echo foo > 2
+  $ echo foo > 3
+  $ echo foo > 4
+  $ echo foo > 5
+  $ hg add *
+
+Build a bogus repository (sparserevlog without general delta)
+
+  $ hg commit -m 'initial changesets'
+  $ echo 'sparserevlog' >> .hg/requires
+  $ for x in `$TESTDIR/seq.py 100`; do
+  >     echo $x >> `expr $x % 5 + 1`
+  >     hg commit -m $x
+  > done
+  $ cd ..