--- a/hgext3rd/evolve/firstmergecache.py	Thu Jul 11 14:31:32 2019 -0700
+++ b/hgext3rd/evolve/firstmergecache.py	Tue Jul 09 10:56:42 2019 -0700
@@ -122,7 +122,7 @@
         else:
             headerdata = data[:self._cachekeysize]
             self._cachekey = self._deserializecachekey(headerdata)
-            self._data.fromstring(data[self._cachekeysize:])
+            compat.arrayfrombytes(self._data, data[self._cachekeysize:])
         self._ondiskkey = self._cachekey
 
     def save(self, repo):
@@ -138,7 +138,7 @@
             cachefile = repo.cachevfs(self._filepath, 'w', atomictemp=True)
             headerdata = self._serializecachekey()
             cachefile.write(headerdata)
-            cachefile.write(self._data.tostring())
+            cachefile.write(compat.arraytobytes(self._data))
             cachefile.close()
             self._ondiskkey = self._cachekey
         except (IOError, OSError) as exc: