diff --git a/src/comp.c b/src/comp.c
index 70a9a64a714..b96fae4ae95 100644
--- a/src/comp.c
+++ b/src/comp.c
@@ -5488,7 +5488,10 @@ native_function_doc (Lisp_Object function)
   if (!VECTORP (cu->data_fdoc_v))
     xsignal2 (Qnative_lisp_file_inconsistent, cu->file,
 	      build_string ("missing documentation vector"));
-  return AREF (cu->data_fdoc_v, XSUBR (function)->doc);
+  EMACS_INT doc = XSUBR (function)->doc;
+  if (doc < 0)
+    return AREF (cu->data_fdoc_v, -doc - 1);
+  return make_fixnum (doc);
 }
 
 static Lisp_Object
@@ -5529,7 +5532,8 @@ make_subr (Lisp_Object symbol_name, Lisp_Object minarg, Lisp_Object maxarg,
   x->s.symbol_name = xstrdup (SSDATA (symbol_name));
   x->s.intspec.native = intspec;
   x->s.command_modes = command_modes;
-  x->s.doc = XFIXNUM (doc_idx);
+  x->s.doc = -XFIXNUM (doc_idx) - 1;
+  eassert (x->s.doc < 0);
 #ifdef HAVE_NATIVE_COMP
   x->s.native_comp_u = comp_u;
   x->s.native_c_name = xstrdup (SSDATA (c_name));
diff --git a/src/doc.c b/src/doc.c
index 88be9121dab..04afe50d3dd 100644
--- a/src/doc.c
+++ b/src/doc.c
@@ -479,7 +479,10 @@ store_function_docstring (Lisp_Object obj, EMACS_INT offset)
     fun = XCDR (fun);
   /* Lisp_Subrs have a slot for it.  */
   if (SUBRP (fun))
-    XSUBR (fun)->doc = offset;
+    {
+      XSUBR (fun)->doc = offset;
+      eassert (XSUBR (fun)->doc >= 0);
+    }
   else if (CLOSUREP (fun))
     {
       /* This bytecode object must have a slot for the docstring, since
diff --git a/src/lisp.h b/src/lisp.h
index 4217dd9e347..4c09afc77e2 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -2228,6 +2228,9 @@ struct Lisp_Subr
       Lisp_Object native;
     } intspec;
     Lisp_Object command_modes;
+    /* positive values: offset into etc/DOC.  Negative values: one's
+       complement of index into the native comp unit's function
+       documentation vector.  */
     EMACS_INT doc;
 #ifdef HAVE_NATIVE_COMP
     Lisp_Object native_comp_u;