diff -r 794afa91f0a5 -r df0873ab5c14 mercurial/scmutil.py
--- a/mercurial/scmutil.py	Thu Aug 02 22:44:41 2018 +0300
+++ b/mercurial/scmutil.py	Fri Apr 13 23:37:53 2018 -0700
@@ -480,8 +480,8 @@
 def isrevsymbol(repo, symbol):
     """Checks if a symbol exists in the repo.
 
-    See revsymbol() for details. Raises error.LookupError if the symbol is an
-    ambiguous nodeid prefix.
+    See revsymbol() for details. Raises error.AmbiguousPrefixLookupError if the
+    symbol is an ambiguous nodeid prefix.
     """
     try:
         revsymbol(repo, symbol)