diff --git a/.travis.yml b/.travis.yml
index 6825260..45d6696 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,5 +2,6 @@ language: nix
 matrix:
   include:
   - script:
-    - nix-shell --run 'cd ofborg && cargo test --lib -- --nocapture'
-    - nix-shell --run 'cd ofborg && cargo test --lib -- --nocapture' --arg useNix1 true
+    - nix-shell --run checkPhase
+    - nix-shell --run checkPhase --arg useNix1 true
+    - nix-build -A ofborg.rs
diff --git a/nix/nixpkgs.json b/nix/nixpkgs.json
index 18ce7f6..f0e97b4 100644
--- a/nix/nixpkgs.json
+++ b/nix/nixpkgs.json
@@ -1,7 +1,7 @@
 {
   "url": "https://github.com/nixos/nixpkgs-channels.git",
-  "rev": "c2fbd472a4ebaae739257a3df93aef25f19dd04f",
-  "date": "2018-02-23T08:08:30+00:00",
-  "sha256": "07mir6nqb98mbykabppgj4dli66h18qsyi4zqp640x22k6bkp2vp",
+  "rev": "2e855dc6b0cd88767a8a5df2faff9e66e1cd7f18",
+  "date": "2018-04-11T10:16:39+08:00",
+  "sha256": "08s6mfh5a05kd2qs3hmza50ng3pyhd3qha4nanwwk0s8fjzqnv4a",
   "fetchSubmodules": true
 }
diff --git a/ofborg/build.rs b/ofborg/build.rs
index 4881c36..58ca368 100644
--- a/ofborg/build.rs
+++ b/ofborg/build.rs
@@ -518,24 +518,27 @@ impl MetricCollector {
         .map(|mtype| {
             let fields: Vec<String> = mtype.enum_field_names();
 
-            let variant_match: String;
-            if fields.len() > 0 {
-                variant_match = format!("{}({})", &mtype.variant(), fields.join(", "));
+            let variant_match = if fields.len() > 0 {
+                format!("{}({})", &mtype.variant(), fields.join(", "))
             } else {
-                variant_match = format!("{}", &mtype.variant());
+                format!("{}", &mtype.variant())
+            };
+
+            let mut index_names: Vec<String> = mtype.enum_index_names();
+            index_names.push("instance".to_owned());
+
+            let mut index_fields = index_names.join(", ");
+            if index_names.len() > 1 {
+                index_fields = format!("({})", index_fields);
             }
 
-            let mut index_fields: Vec<String> = mtype.enum_index_names();
-            index_fields.push("instance".to_owned());
-
-
             format!("
       Event::{} => {{
         let mut accum_table = self.{}
           .lock()
           .expect(\"Failed to unwrap metric mutex for {}\");
         let accum = accum_table
-          .entry(({}))
+          .entry({})
           .or_insert(0);
         *accum += {};
       }}
@@ -543,7 +546,7 @@ impl MetricCollector {
                     variant_match,
                     &mtype.metric_name(),
                     &mtype.metric_name(),
-                    index_fields.join(", "),
+                    index_fields,
                     &mtype.record_value(),
             )
         }).collect();
diff --git a/shell.nix b/shell.nix
index 9843aec..5a35ebe 100644
--- a/shell.nix
+++ b/shell.nix
@@ -15,7 +15,7 @@ let
       curl
       bash
     ]
-      ++ stdenv.lib.optional useNix1 nix;
+      ++ stdenv.lib.optional useNix1 nix1;
 
     # HISTFILE = "${src}/.bash_hist";
   };
@@ -32,10 +32,18 @@ let
       pkgconfig
       git
     ]
-      ++ stdenv.lib.optional useNix1 nix
+      ++ stdenv.lib.optional useNix1 nix1
       ++ stdenv.lib.optional stdenv.isDarwin pkgs.darwin.Security;
 
+    postHook = ''
+      checkPhase() {
+          ( cd "${builtins.toString ./.}/ofborg" && cargo test --lib )
+      }
+    '';
+
     HISTFILE = "${toString ./.}/.bash_hist";
+    RUSTFLAGS = "-D warnings";
+
     passthru.phpEnv = phpEnv;
   };