--- a/mercurial/scmutil.py	Sun Feb 04 20:46:26 2018 +0530
+++ b/mercurial/scmutil.py	Mon Feb 05 12:58:51 2018 +0530
@@ -1196,7 +1196,7 @@
             if k == self.firstlinekey:
                 e = "key name '%s' is reserved" % self.firstlinekey
                 raise error.ProgrammingError(e)
-            if not k[0].isalpha():
+            if not k[0:1].isalpha():
                 e = "keys must start with a letter in a key-value file"
                 raise error.ProgrammingError(e)
             if not k.isalnum():