diff --git a/infer/src/base/SourceFile.ml b/infer/src/base/SourceFile.ml
index 6615dcf99..b05c6fdf3 100644
--- a/infer/src/base/SourceFile.ml
+++ b/infer/src/base/SourceFile.ml
@@ -33,13 +33,6 @@ module Map = Caml.Map.Make (OrderedSourceFile)
 
 module Set = Caml.Set.Make (OrderedSourceFile)
 
-let rel_path_from_abs_path root fname =
-  let relative_complemented_fname = Utils.filename_to_relative root fname in
-  if String.is_prefix ~prefix:root fname &&
-     Filename.is_relative relative_complemented_fname then
-    Some relative_complemented_fname
-  else None (* The project root is not a prefix of the file name *)
-
 let from_abs_path fname =
   if Filename.is_relative fname then
     (failwithf
@@ -49,10 +42,10 @@ let from_abs_path fname =
   let fname_real = try Utils.realpath fname with Unix.Unix_error _ -> fname in
   let project_root_real = Utils.realpath Config.project_root in
   let models_dir_real = Config.models_src_dir in
-  match rel_path_from_abs_path project_root_real fname_real with
+  match Utils.filename_to_relative ~root:project_root_real fname_real with
   | Some path -> RelativeProjectRoot path
   | None -> (
-      match rel_path_from_abs_path models_dir_real fname_real with
+      match Utils.filename_to_relative ~root:models_dir_real fname_real with
       | Some path -> RelativeInferModel path
       | None -> Absolute fname (* fname is absolute already *)
     )
diff --git a/infer/src/base/Utils.ml b/infer/src/base/Utils.ml
index 2e1e62d09..9d626692e 100644
--- a/infer/src/base/Utils.ml
+++ b/infer/src/base/Utils.ml
@@ -120,13 +120,13 @@ let filename_to_absolute ~root fname =
 
 
 (** Convert an absolute filename to one relative to the given directory. *)
-let filename_to_relative root fname =
+let filename_to_relative ~root fname =
   let rec relativize_if_under origin target =
     match origin, target with
     | x :: xs, y :: ys when x = y -> relativize_if_under xs ys
-    | [], [] -> "."
-    | [], ys -> Filename.of_parts ys
-    | _ -> fname
+    | [], [] -> Some "."
+    | [], ys -> Some (Filename.of_parts ys)
+    | _ -> None
   in
   relativize_if_under (Filename.parts root) (Filename.parts fname)
 
diff --git a/infer/src/base/Utils.mli b/infer/src/base/Utils.mli
index 90c93d002..884cf7730 100644
--- a/infer/src/base/Utils.mli
+++ b/infer/src/base/Utils.mli
@@ -32,8 +32,9 @@ val read_file : string -> string list option
 (** Convert a filename to an absolute one if it is relative, and normalize "." and ".." *)
 val filename_to_absolute : root:string -> string -> string
 
-(** Convert an absolute filename to one relative to a root directory *)
-val filename_to_relative : string -> string -> string
+(** Convert an absolute filename to one relative to a root directory.  Returns [None] if filename is
+    not under root. *)
+val filename_to_relative : root:string -> string -> string option
 
 (** type for files used for printing *)
 type outfile =