diff --git a/tools/compiler_version.ml b/tools/compiler_version.ml
index f675a20..ee4169b 100644
--- a/tools/compiler_version.ml
+++ b/tools/compiler_version.ml
@@ -137,11 +137,15 @@ let known_versions =
 
 let is_known v = List.mem v known_versions
 
+let is_known_release {major; minor; _} =
+  List.exists (fun { major = major'; minor = minor'; _ } ->
+    major = major' && minor = minor') known_versions
+
 let is_development _ = false
 
 let make x y z extra_info =
   let v = mk x y z in
-  if is_known v then { v with extra_info } else raise Not_found
+  { v with extra_info }
 
 let to_string { major; minor; patch_level; extra_info } =
   let printer = Printf.sprintf in
diff --git a/tools/compiler_version.mli b/tools/compiler_version.mli
index c3ab3d6..7c272fd 100644
--- a/tools/compiler_version.mli
+++ b/tools/compiler_version.mli
@@ -56,6 +56,8 @@ val known_versions : t list
 
 val is_known : t -> bool
 
+val is_known_release : t -> bool
+
 val is_development : t -> bool
 
 val major : t -> int
diff --git a/tools/stdcompatpp.mll b/tools/stdcompatpp.mll
index 57914b1..9b49a2b 100644
--- a/tools/stdcompatpp.mll
+++ b/tools/stdcompatpp.mll
@@ -27,8 +27,8 @@ type options = {
   debug : bool;
 }
 
-let init_options () = {
-  compiler_version = Compiler_version.of_string Sys.ocaml_version;
+let init_options compiler_version = {
+  compiler_version;
   source_type = OCaml;
   debug = false;
 }
@@ -40,11 +40,11 @@ let dprintf options fmt =
   printer stderr fmt
 
 let set_compiler_version opts version =
-  let compiler_version =
-    try Compiler_version.of_string version with
-    _ -> unknown "compiler version" version
-  in
-  opts := { !opts with compiler_version  }
+  let compiler_version = Compiler_version.of_string version in
+  if Compiler_version.is_known compiler_version then
+    opts := { !opts with compiler_version  }
+  else
+    unknown "compiler version" version
 
 let set_source_type opts source_type =
   let source_type =
@@ -339,7 +339,14 @@ and c_lexer options state = parse
 {
 
 let main () =
-  let options = init_options () |> parse_commandline in
+  let compiler_version = Compiler_version.of_string Sys.ocaml_version in
+  let compiler_version =
+    if Compiler_version.is_known_release compiler_version then
+      compiler_version
+    else
+      List.hd (List.rev Compiler_version.known_versions)
+  in
+  let options = init_options compiler_version |> parse_commandline in
   let lexer = match options.source_type with
     | OCaml -> ocaml_lexer
     | C -> c_lexer