diff --git a/src/blib2to3/pgen2/tokenize.py b/src/blib2to3/pgen2/tokenize.py
index a6353d154c9..35dedee7008 100644
--- a/src/blib2to3/pgen2/tokenize.py
+++ b/src/blib2to3/pgen2/tokenize.py
@@ -163,7 +163,6 @@ def _combinations(*l):
     '"""': double3prog,
     **{f"{prefix}'''": single3prog for prefix in _strprefixes},
     **{f'{prefix}"""': double3prog for prefix in _strprefixes},
-    **{prefix: None for prefix in _strprefixes},
 }
 
 triple_quoted: Final = (
@@ -601,9 +600,10 @@ def generate_tokens(
                         strstart = (lnum, start)
                         endprog = (
                             endprogs[initial]
-                            or endprogs[token[1]]
-                            or endprogs[token[2]]
+                            or endprogs.get(token[1])
+                            or endprogs.get(token[2])
                         )
+                        assert endprog is not None, f"endprog not found for {token}"
                         contstr, needcont = line[start:], 1
                         contline = line
                         break