diff --git a/solver.ml b/solver.ml
index e5c03aec157bf2f9fbdfc6cbcf7125c92aef59cc..d23c0eae322762249e081f6b941431cc0d2b8249 100644
--- a/solver.ml
+++ b/solver.ml
@@ -302,7 +302,7 @@ let build_command ~raw_options confg_solver property model =
       Stdlib.close_out out_channel;
       Ok tmp
     with Sys_error e ->
-      Error (Format.sprintf "Unexpected failure@ `%s'." e)
+      Error (Format.sprintf "Unexpected failure:@ %s." e)
   end >>= fun prop ->
   (* Build actual command-line. *)
   try
@@ -335,7 +335,7 @@ let build_command ~raw_options confg_solver property model =
     in
     Ok (prop, cmd)
   with Failure e ->
-    Error (Format.sprintf "Unexpected failure@ `%s'." e)
+    Error (Format.sprintf "Unexpected failure:@ %s." e)
 
 let exec ~raw_options config model property =
   let open Result in