--- a/mercurial/phases.py	Thu May 24 09:59:10 2018 +0200
+++ b/mercurial/phases.py	Tue May 29 18:13:19 2018 +0200
@@ -140,6 +140,10 @@
 remotehiddenphases = tuple(allphases[2:])
 localhiddenphases = tuple(p for p in allphases if p & HIDEABLE_FLAG)
 
+def supportinternal(repo):
+    """True if the internal phase can be used on a repository"""
+    return 'internal-phase' in repo.requirements
+
 def _readroots(repo, phasedefaults=None):
     """Read phase roots from disk
 
@@ -442,6 +446,9 @@
     def _retractboundary(self, repo, tr, targetphase, nodes):
         # Be careful to preserve shallow-copied values: do not update
         # phaseroots values, replace them.
+        if targetphase == internal and not supportinternal(repo):
+            msg = 'this repository does not support the internal phase'
+            raise error.ProgrammingError(msg)
 
         repo = repo.unfiltered()
         currentroots = self.phaseroots[targetphase]

--- a/tests/test-phases.t	Thu May 24 09:59:10 2018 +0200
+++ b/tests/test-phases.t	Tue May 29 18:13:19 2018 +0200
@@ -832,8 +832,38 @@
 Test for the "internal" phase
 =============================
 
-  $ hg init internal-phase
+Check we deny its usage on older repository
+
+  $ hg init no-internal-phase --config format.internal-phase=no
+  $ cd no-internal-phase
+  $ cat .hg/requires
+  dotencode
+  fncache
+  generaldelta
+  revlogv1
+  store
+  $ echo X > X
+  $ hg add X
+  $ hg status
+  A X
+  $ hg --config "phases.new-commit=internal" commit -m "my test internal commit" 2>&1 | grep ProgrammingError
+  ** ProgrammingError: this repository does not support the internal phase
+      raise error.ProgrammingError(msg)
+  mercurial.error.ProgrammingError: this repository does not support the internal phase
+
+  $ cd ..
+
+Check it works fine with repository that supports it.
+
+  $ hg init internal-phase --config format.internal-phase=yes
   $ cd internal-phase
+  $ cat .hg/requires
+  dotencode
+  fncache
+  generaldelta
+  internal-phase
+  revlogv1
+  store
   $ mkcommit A
   test-debug-phase: new rev 0:  x -> 1
   test-hook-close-phase: 4a2df7238c3b48766b5e22fafbb8a2f506ec8256:   -> draft