--- i/src/syntax.c
+++ w/src/syntax.c
@@ -3192,7 +3192,11 @@ scan_sexps_forward (struct lisp_parse_state *state,
 
   while (from < end)
     {
-      if (SYNTAX_FLAGS_COMSTART_FIRST (prev_from_syntax)
+      INC_FROM;
+      code = prev_from_syntax & 0xff;
+
+      if (from < end
+          && SYNTAX_FLAGS_COMSTART_FIRST (prev_from_syntax)
 	  && (c1 = FETCH_CHAR (from_byte),
 	      syntax = SYNTAX_WITH_FLAGS (c1),
 	      SYNTAX_FLAGS_COMSTART_SECOND (syntax)))
@@ -3213,8 +3217,6 @@ scan_sexps_forward (struct lisp_parse_state *state,
 	}
       else
         {
-          INC_FROM;
-          code = prev_from_syntax & 0xff;
           if (code == Scomment_fence)
             {
               /* Record the comment style we have entered so that only