--- a/third_party/rust/app_units/.cargo-checksum.json
+++ b/third_party/rust/app_units/.cargo-checksum.json
@@ -1,1 +1,1 @@
-{"files":{".cargo-ok":"e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",".travis.yml":"6b96b2c6bfd7e1acef4b825a2813fc4277859eb9400a16800db8835c25e4087d","Cargo.toml":"5b797a2d0056ba56fd8683ee2c288d15b4f39f56f5e2c76adbb0ba9a34dbdf09","README.md":"9f048d969f9f8333cdcdb892744cd0816e4f2922c8817fa5e9e07f9472fe1050","src/app_unit.rs":"c091314a70a4dc750b98eb937b00b42c6f6f1333ca6154a995f08ba5627bbb93","src/lib.rs":"2df7d863c47d8b22f9af66caeafa87e6a206ee713a8aeaa55c5a80a42a92513b"},"package":"99f3af85d0c7c054d95da6405117b523284a97484494b44a6dec58b9617eabf6"}
\ No newline at end of file
+{"files":{".cargo-ok":"e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",".travis.yml":"6b96b2c6bfd7e1acef4b825a2813fc4277859eb9400a16800db8835c25e4087d","Cargo.toml":"1399354caef7c9cee1da231854345daef0c395050605074397b12ef4491a4825","README.md":"9f048d969f9f8333cdcdb892744cd0816e4f2922c8817fa5e9e07f9472fe1050","src/app_unit.rs":"13f8c33b7ea85943a81a3866d8ad156793ae0723f2e44b7ca9bb5497e8215354","src/lib.rs":"2df7d863c47d8b22f9af66caeafa87e6a206ee713a8aeaa55c5a80a42a92513b"},"package":"b502f30531df49d388ac6efbc8fb25652d54df5cc2bca653361f683c6dd2f075"}
\ No newline at end of file

--- a/third_party/rust/app_units/Cargo.toml
+++ b/third_party/rust/app_units/Cargo.toml
@@ -7,25 +7,25 @@
 #
 # If you believe there's an error in this file please file an
 # issue against the rust-lang/cargo repository. If you're
 # editing this file be aware that the upstream Cargo.toml
 # will likely look very different (and much more reasonable)
 
 [package]
 name = "app_units"
-version = "0.5.0"
+version = "0.5.1"
 authors = ["The Servo Project Developers"]
 description = "Servo app units type (Au)"
 documentation = "http://doc.servo.org/app_units/"
 license = "MPL-2.0"
 repository = "https://github.com/servo/app_units"
-[dependencies.rustc-serialize]
-version = "0.3"
+[dependencies.heapsize]
+version = ">=0.3, < 0.5"
 
 [dependencies.num-traits]
 version = "0.1.32"
 
+[dependencies.rustc-serialize]
+version = "0.3"
+
 [dependencies.serde]
 version = "1.0"
-
-[dependencies.heapsize]
-version = ">=0.3, < 0.5"

--- a/third_party/rust/app_units/src/app_unit.rs
+++ b/third_party/rust/app_units/src/app_unit.rs
@@ -54,20 +54,20 @@ impl Zero for Au {
     }
 
     #[inline]
     fn is_zero(&self) -> bool {
         self.0 == 0
     }
 }
 
-// 1 << 30 lets us add/subtract two Au and check for overflow
+// (1 << 30) - 1 lets us add/subtract two Au and check for overflow
 // after the operation. Gecko uses the same min/max values
-pub const MAX_AU: Au = Au(1 << 30);
-pub const MIN_AU: Au = Au(- (1 << 30));
+pub const MAX_AU: Au = Au((1 << 30) - 1);
+pub const MIN_AU: Au = Au(- ((1 << 30) - 1));
 
 impl Encodable for Au {
     fn encode<S: Encoder>(&self, e: &mut S) -> Result<(), S::Error> {
         e.emit_f64(self.to_f64_px())
     }
 }
 
 impl fmt::Debug for Au {