src/Pure/PIDE/resources.scala

   def check_thy_reader(node_name: Document.Node.Name, reader: Reader[Char],
     start: Token.Pos = Token.Pos.command, strict: Boolean = true): Document.Node.Header =
   {
     if (node_name.is_theory && reader.source.length > 0) {
       try {
-        val header = Thy_Header.read(reader, start, strict)
+        val header = Thy_Header.read(reader, start, strict).check_keywords
 
         val base_name = node_name.theory_base_name
         if (Long_Name.is_qualified(header.name)) {
           error("Bad theory name " + quote(header.name) + " with qualification" +
             Position.here(header.pos))