diff --git a/src/Cron.fs b/src/Cron.fs
index 96528b4..0d27168 100644
--- a/src/Cron.fs
+++ b/src/Cron.fs
@@ -9,6 +9,7 @@ open Datatypes
 
 open Pentole.String
 open Pentole.Path
+open Pentole.Map
 
 type User = string
 
@@ -78,10 +79,6 @@ type JobKey = {
 let sort_cron_jobs (now: Instant) (db_crons: Database.Requirements seq) =
 
 
-    let all_jobs  =
-        db_crons
-        |> ResultList.collect (parse now)
-
     let build_deps (lst: CronJob list) =
         let standalone, deps =
             List.partition (function | {when_=Cron _} -> true | _ -> false) lst
@@ -93,18 +90,23 @@ let sort_cron_jobs (now: Instant) (db_crons: Database.Requirements seq) =
          in index, deps  
     in
     let rec build_job_table acc (index: Map<JobKey, CronJob>) = function
-        | [] -> acc
+        | [] -> Ok acc
+        | {when_=Cron _}::_ -> invalidOp "The jobs should have been partitioned"
         | {when_=Pattern (After jb)} as x::xs ->
             let father = {j=jb; h=x.hostname}
             let previous =
                 match Map.tryFind father acc with
-                // TODO: error handling 
-                | None -> Map.find father index |> List.singleton
-                | Some p -> p
-            let acc' = Map.add father (x::previous) acc
-            build_job_table acc' index xs
-        | {when_=Cron _}::_ -> failwith "TODO" // TODO
+                | None ->
+                    Map.find father index |> Result.map List.singleton
+                | Some p -> Ok p
+            if previous |> Result.isError then
+                Error "Invalid job definition. No such job_name {jb} in host {x.hostname}"
+            else
+                let p = Result.get previous
+                let acc' = Map.add father (x::p) acc
+                build_job_table acc' index xs
 
-    all_jobs
+    db_crons
+    |> ResultList.collect (parse now)
     |> Result.map build_deps
     |> Result.map (fun (standalone, deps) -> build_job_table Map.empty standalone deps)
diff --git a/src/Map.fs b/src/Map.fs
new file mode 100644
index 0000000..2ab33c2
--- /dev/null
+++ b/src/Map.fs
@@ -0,0 +1,7 @@
+namespace Pentole
+
+module Map =
+    let find (key: 'key) (map: Map<'key, 'value>) =
+        match Map.tryFind key map with
+        | Some v -> Ok v
+        | None -> Error $"Can't find key {key}"
diff --git a/src/Result.fs b/src/Result.fs
index a60e265..03cefa8 100644
--- a/src/Result.fs
+++ b/src/Result.fs
@@ -21,6 +21,14 @@ module Result =
         | Error e, _ -> Error e
         | _, Error e -> Error e
 
+    let inline get r =
+        match r with
+        | Ok o -> o
+        | Error e ->
+             $"Tried to access value from Result r, error={e}"
+             |> System.ArgumentException
+             |> raise
+
 
     type ToStringWrapper(toString) =
         override _.ToString() = toString ()
diff --git a/src/src.fsproj b/src/src.fsproj
index d0436bf..72bc45a 100644
--- a/src/src.fsproj
+++ b/src/src.fsproj
@@ -8,6 +8,7 @@
   <ItemGroup>
     <Compile Include="Environment.fs" />
     <Compile Include="LoggingHelpers.fs" />
+    <Compile Include="Map.fs" />
     <Compile Include="Result.fs" />
     <Compile Include="String.fs" />
     <Compile Include="Datatypes.fs" />