diff --git a/bin/flake_env.re b/bin/flake_env.re
index 4567cb9..d2b6dfd 100644
--- a/bin/flake_env.re
+++ b/bin/flake_env.re
@@ -12,7 +12,14 @@ let main = () => {
       switch (Lib.Watches.get()) {
       | Ok(watches) =>
         let paths = Array.map(~f=watch => watch.path, watches);
-        let hash = Util.hash_files(paths);
+        let hash =
+          switch (Util.hash_files(paths)) {
+          | Ok(hsh) => hsh
+          | Error(msg) =>
+            Printf.eprintf("%s\n", e);
+            exit(1);
+          };
+
         let profile = layout_directory ++ "/flake-profile-" ++ hash;
         let profile_rc = profile ++ ".rc";
 
diff --git a/lib/flake_env__util.re b/lib/flake_env__util.re
index 1b5aa7b..e471010 100644
--- a/lib/flake_env__util.re
+++ b/lib/flake_env__util.re
@@ -17,23 +17,39 @@ let nix = args =>
   );
 
 let hash_files = filenames => {
-  /*** Hash all entries in [filenames], returning a hex-encoded string of the hash of their contents */
+  /*** Hash all entries in [filenames]
+         Returns Some(hex-string) or None if no filenames are found.
+       */
   let ctx = Sha1.init();
-  let () =
+  let files_to_hash =
     filenames
     |> Array.filter(~f=f =>
          switch (Sys_unix.file_exists(f)) {
          | `Yes => true
-         | _ => false
+         | _ =>
+           // let fullpth = Filename_unix.realpath(f);
+           Printf.eprintf(
+             "Cannot find file %s (cwd: %s)\n",
+             f,
+             Core_unix.getcwd(),
+           );
+           false;
          }
-       )
-    |> Array.iter(~f=f => {
-         f
-         |> In_channel.create
-         |> In_channel.input_all
-         |> Sha1.update_string(ctx)
-       });
-  Sha1.finalize(ctx) |> Sha1.to_hex;
+       );
+
+  switch (files_to_hash |> Array.length) {
+  | 0 => Error("No files found to hash")
+  | _ =>
+    let () =
+      files_to_hash
+      |> Array.iter(~f=f => {
+           f
+           |> In_channel.create
+           |> In_channel.input_all
+           |> Sha1.update_string(ctx)
+         });
+    Ok(Sha1.finalize(ctx) |> Sha1.to_hex);
+  };
 };
 
 let rec rmrf = path => {
diff --git a/tests/flake_env_test_util.re b/tests/flake_env_test_util.re
index e45ccaf..9444d8a 100644
--- a/tests/flake_env_test_util.re
+++ b/tests/flake_env_test_util.re
@@ -16,7 +16,22 @@ let _syst_to_bool =
 
 let check_exit_or_signal =
   Alcotest.(check(Alcotest.pair(testable_exit_or_signal, string)));
-let check_string = Alcotest.(check(string));
+let testable_result_string =
+  Alcotest.testable(
+    (pp_fmt, elem) => {
+      switch (elem) {
+      | Ok(s) => Fmt.pf(pp_fmt, "Ok(%s)", s)
+      | Error(s) => Fmt.pf(pp_fmt, "Error(%s)", s)
+      }
+    },
+    (a, b) =>
+      switch (a, b) {
+      | (Ok(a), Ok(b)) => String.compare(a, b) == 0
+      | (Error(a), Error(b)) => String.compare(a, b) == 0
+      | _ => false
+      },
+  );
+let check_result_string = Alcotest.(check(testable_result_string));
 let check_bool = Alcotest.(check(bool));
 let check_get_args =
   Alcotest.(
@@ -47,26 +62,26 @@ let test_run_process_stdout = () =>
   );
 
 let test_hash_one = () => {
-  check_string(
+  check_result_string(
     "Hash matches",
-    hash_files([|"../LICENSE"|]),
-    "b43cf2e824eb66ba0e8f939c08072a8e307b5e5f",
+    Ok("32b4ac64be805d730745f6bac45a5d95174ebd10"),
+    hash_files([|"spit_version.sh"|]),
   );
 };
 
 let test_hash_multiple = () => {
-  check_string(
+  check_result_string(
     "Hash matches",
-    hash_files([|"../LICENSE", "../LICENSE"|]),
-    "08304d8baeed02722f81252952b00f6ac011ce0c",
+    Ok("e4c880fc6ab9a1b88e6be18e53fc4cec9f463d1a"),
+    hash_files([|"spit_version.sh", "spit_version.sh"|]),
   );
 };
 
 let test_hash_filters_nonexistent = () => {
-  check_string(
+  check_result_string(
     "Hash matches",
-    hash_files([|"../LICENSE", "FOOBARBAZ"|]),
-    "b43cf2e824eb66ba0e8f939c08072a8e307b5e5f",
+    Ok("32b4ac64be805d730745f6bac45a5d95174ebd10"),
+    hash_files([|"spit_version.sh", "FOOBARBAZ"|]),
   );
 };