diff -r e8f27a35ee0f -r b74b9d0bf763 src/Pure/Isar/outer_syntax.scala
--- a/src/Pure/Isar/outer_syntax.scala	Sat Oct 07 12:50:05 2017 +0200
+++ b/src/Pure/Isar/outer_syntax.scala	Sat Oct 07 13:13:46 2017 +0200
@@ -18,9 +18,7 @@
 
   def init(): Outer_Syntax = new Outer_Syntax(completion = Completion.init())
 
-  def merge(syns: List[Outer_Syntax]): Outer_Syntax =
-    if (syns.isEmpty) Thy_Header.bootstrap_syntax
-    else (syns.head /: syns.tail)(_ ++ _)
+  def merge(syns: List[Outer_Syntax]): Outer_Syntax = (empty /: syns)(_ ++ _)
 
 
   /* string literals */
@@ -109,6 +107,7 @@
 
   def ++ (other: Outer_Syntax): Outer_Syntax =
     if (this eq other) this
+    else if (this eq Outer_Syntax.empty) other
     else {
       val keywords1 = keywords ++ other.keywords
       val completion1 = completion ++ other.completion