diff --git a/Changelog.md b/Changelog.md
--- a/Changelog.md
+++ b/Changelog.md
@@ -1,3 +1,13 @@
+## 1.2.21 -- 2018-08-12
+
+### Other Changes
+
+* Increased the upper-bound of `aeson` to < 1.5.
+* Increased the upper-bound of `base` to < 4.12.
+* Increased the upper-bound of `errors` to < 2.4.
+* Increased the upper-bound of `free` to < 5.2.
+* Increased the upper-bound of `mwc-random` to < 0.15.
+
 ## 1.2.20
 
 ### Other Changes
diff --git a/engine-io.cabal b/engine-io.cabal
--- a/engine-io.cabal
+++ b/engine-io.cabal
@@ -1,5 +1,5 @@
 name: engine-io
-version: 1.2.20
+version: 1.2.21
 synopsis: A Haskell implementation of Engine.IO
 homepage: http://github.com/ocharles/engine.io
 license: BSD3
@@ -27,16 +27,16 @@
    RankNTypes, OverloadedStrings, RecordWildCards
 
   build-depends:
-    aeson >=0.7 && <1.4,
+    aeson >=0.7 && <1.5,
     async >=2.0 && <2.3,
     attoparsec >=0.11 && <0.14,
-    base >=4.6 && <4.11,
+    base >=4.6 && <4.12,
     base64-bytestring >=1.0 && <1.1,
     bytestring >=0.10.2.0 && <0.11,
-    errors >= 2.0.0 && <2.3,
-    free >= 4.9 && <5.1,
+    errors >= 2.0.0 && <2.4,
+    free >= 4.9 && <5.2,
     monad-loops >=0.4 && <0.5,
-    mwc-random >=0.13 && <0.14,
+    mwc-random >=0.13 && <0.15,
     stm >=2.4 && <2.5,
     stm-delay >= 0.1.1 && <0.2,
     text >=0.11 && <1.3,