diff --git a/js_of_ast.ml b/js_of_ast.ml
index ae2a39c..b28fc0f 100644
--- a/js_of_ast.ml
+++ b/js_of_ast.ml
@@ -123,8 +123,9 @@ let is_infix f args = match args with
      let open Lexing in
      if f.exp_loc.loc_ghost then false else
      if x.exp_loc.loc_ghost then false else
-       x.exp_loc.loc_end.pos_lnum < f.exp_loc.loc_start.pos_lnum ||
-       x.exp_loc.loc_end.pos_cnum < f.exp_loc.loc_start.pos_cnum
+       x.exp_loc.loc_start.pos_lnum < f.exp_loc.loc_start.pos_lnum ||
+       (x.exp_loc.loc_start.pos_lnum = f.exp_loc.loc_start.pos_lnum &&
+        x.exp_loc.loc_start.pos_cnum < f.exp_loc.loc_start.pos_cnum)
 
 exception Map_fields_elements_mismatch_number_args
 
diff --git a/tests/arith.ml b/tests/arith.ml
index d5ee32b..0e62112 100644
--- a/tests/arith.ml
+++ b/tests/arith.ml
@@ -1 +1,3 @@
 let myadd x y = x +. y
+
+let subst x = x-1