diff --git a/package.json b/package.json
index ec4892a..9da5ebc 100644
--- a/package.json
+++ b/package.json
@@ -24,7 +24,7 @@
     "yargs": "^6.3.0"
   },
   "devDependencies": {
-    "chai": "^3.5.0",
+    "chai": "^4.2.0",
     "mocha": "^3.2.0",
     "np": "^2.12.0",
     "sinon": "^2.0.0"
diff --git a/yarn.lock b/yarn.lock
index 47c129d..2e7f163 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -67,9 +67,9 @@ assert-plus@^0.2.0:
   version "0.2.0"
   resolved "https://registry.yarnpkg.com/assert-plus/-/assert-plus-0.2.0.tgz#d74e1b87e7affc0db8aadb7021f3fe48101ab234"
 
-assertion-error@^1.0.1:
-  version "1.0.2"
-  resolved "https://registry.yarnpkg.com/assertion-error/-/assertion-error-1.0.2.tgz#13ca515d86206da0bac66e834dd397d87581094c"
+assertion-error@^1.1.0:
+  version "1.1.0"
+  resolved "https://registry.yarnpkg.com/assertion-error/-/assertion-error-1.1.0.tgz#e60b6b0e8f301bd97e5375215bda406c85118c0b"
 
 async@^2.0.1, async@~2.0.1:
   version "2.0.1"
@@ -203,13 +203,16 @@ caseless@~0.12.0:
   version "0.12.0"
   resolved "https://registry.yarnpkg.com/caseless/-/caseless-0.12.0.tgz#1b681c21ff84033c826543090689420d187151dc"
 
-chai@^3.5.0:
-  version "3.5.0"
-  resolved "https://registry.yarnpkg.com/chai/-/chai-3.5.0.tgz#4d02637b067fe958bdbfdd3a40ec56fef7373247"
+chai@^4.2.0:
+  version "4.2.0"
+  resolved "https://registry.yarnpkg.com/chai/-/chai-4.2.0.tgz#760aa72cf20e3795e84b12877ce0e83737aa29e5"
   dependencies:
-    assertion-error "^1.0.1"
-    deep-eql "^0.1.3"
-    type-detect "^1.0.0"
+    assertion-error "^1.1.0"
+    check-error "^1.0.2"
+    deep-eql "^3.0.1"
+    get-func-name "^2.0.0"
+    pathval "^1.1.0"
+    type-detect "^4.0.5"
 
 chalk@^1.0.0, chalk@^1.1.1, chalk@^1.1.3:
   version "1.1.3"
@@ -221,6 +224,10 @@ chalk@^1.0.0, chalk@^1.1.1, chalk@^1.1.3:
     strip-ansi "^3.0.0"
     supports-color "^2.0.0"
 
+check-error@^1.0.2:
+  version "1.0.2"
+  resolved "https://registry.yarnpkg.com/check-error/-/check-error-1.0.2.tgz#574d312edd88bb5dd8912e9286dd6c0aed4aac82"
+
 cli-boxes@^1.0.0:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/cli-boxes/-/cli-boxes-1.0.0.tgz#4fa917c3e59c94a004cd61f8ee509da651687143"
@@ -364,11 +371,11 @@ decamelize@^1.1.1, decamelize@^1.1.2:
   version "1.2.0"
   resolved "https://registry.yarnpkg.com/decamelize/-/decamelize-1.2.0.tgz#f6534d15148269b20352e7bee26f501f9a191290"
 
-deep-eql@^0.1.3:
-  version "0.1.3"
-  resolved "https://registry.yarnpkg.com/deep-eql/-/deep-eql-0.1.3.tgz#ef558acab8de25206cd713906d74e56930eb69f2"
+deep-eql@^3.0.1:
+  version "3.0.1"
+  resolved "https://registry.yarnpkg.com/deep-eql/-/deep-eql-3.0.1.tgz#dfc9404400ad1c8fe023e7da1df1c147c4b444df"
   dependencies:
-    type-detect "0.1.1"
+    type-detect "^4.0.0"
 
 deep-extend@~0.4.0:
   version "0.4.1"
@@ -550,6 +557,10 @@ get-caller-file@^1.0.1:
   version "1.0.2"
   resolved "https://registry.yarnpkg.com/get-caller-file/-/get-caller-file-1.0.2.tgz#f702e63127e7e231c160a80c1554acb70d5047e5"
 
+get-func-name@^2.0.0:
+  version "2.0.0"
+  resolved "https://registry.yarnpkg.com/get-func-name/-/get-func-name-2.0.0.tgz#ead774abee72e20409433a066366023dd6887a41"
+
 get-stdin@^4.0.1:
   version "4.0.1"
   resolved "https://registry.yarnpkg.com/get-stdin/-/get-stdin-4.0.1.tgz#b968c6b0a04384324902e8bf1a5df32579a450fe"
@@ -1395,6 +1406,10 @@ path-type@^2.0.0:
   dependencies:
     pify "^2.0.0"
 
+pathval@^1.1.0:
+  version "1.1.0"
+  resolved "https://registry.yarnpkg.com/pathval/-/pathval-1.1.0.tgz#b942e6d4bde653005ef6b71361def8727d0645e0"
+
 performance-now@^0.2.0:
   version "0.2.0"
   resolved "https://registry.yarnpkg.com/performance-now/-/performance-now-0.2.0.tgz#33ef30c5c77d4ea21c5a53869d91b56d8f2555e5"
@@ -1859,17 +1874,9 @@ tweetnacl@^0.14.3, tweetnacl@~0.14.0:
   version "0.14.5"
   resolved "https://registry.yarnpkg.com/tweetnacl/-/tweetnacl-0.14.5.tgz#5ae68177f192d4456269d108afa93ff8743f4f64"
 
-type-detect@0.1.1:
-  version "0.1.1"
-  resolved "https://registry.yarnpkg.com/type-detect/-/type-detect-0.1.1.tgz#0ba5ec2a885640e470ea4e8505971900dac58822"
-
-type-detect@^1.0.0:
-  version "1.0.0"
-  resolved "https://registry.yarnpkg.com/type-detect/-/type-detect-1.0.0.tgz#762217cc06db258ec48908a1298e8b95121e8ea2"
-
-type-detect@^4.0.0:
-  version "4.0.0"
-  resolved "https://registry.yarnpkg.com/type-detect/-/type-detect-4.0.0.tgz#62053883542a321f2f7b25746dc696478b18ff6b"
+type-detect@^4.0.0, type-detect@^4.0.5:
+  version "4.0.8"
+  resolved "https://registry.yarnpkg.com/type-detect/-/type-detect-4.0.8.tgz#7646fb5f18871cfbb7749e69bd39a6388eb7450c"
 
 typedarray@~0.0.5:
   version "0.0.6"