diff --git a/.travis.yml b/.travis.yml
index ea7e73c..aab9d27 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,25 +7,37 @@ matrix:
     - dist: precise
       php: "5.3"
       env: USE_PSALM=0
+           CHECK_MBSTRING=1
     - dist: precise
       php: "5.4"
       env: USE_PSALM=0
+           CHECK_MBSTRING=1
     - dist: precise
       php: "5.5"
       env: USE_PSALM=0
+           CHECK_MBSTRING=1
     - dist: precise
       php: "5.6"
       env: USE_PSALM=1
+           CHECK_MBSTRING=1
     - php: "7.0"
       env: USE_PSALM=1
+           CHECK_MBSTRING=1
     - php: "7.1"
       env: USE_PSALM=1
+           CHECK_MBSTRING=1
     - php: "7.2"
       env: USE_PSALM=1
+           CHECK_MBSTRING=0
+    - php: "nightly"
+      env: USE_PSALM=1
+           CHECK_MBSTRING=0
     - php: "hhvm"
       env: USE_PSALM=1
+           CHECK_MBSTRING=1
     - php: "master"
       env: USE_PSALM=1
+           CHECK_MBSTRING=0
   allow_failures:
     - php: "hhvm"
     - php: "master"
diff --git a/composer.json b/composer.json
index 31440d8..1c5978c 100644
--- a/composer.json
+++ b/composer.json
@@ -24,8 +24,7 @@
     "php": ">=5.2.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "4.*|5.*",
-    "vimeo/psalm": "dev-master"
+    "phpunit/phpunit": "4.*|5.*"
   },
   "suggest": {
     "ext-libsodium": "Provides a modern crypto API that can be used to generate random bytes."