diff --git a/stack.yaml b/stack.yaml
index d395b87..a28329a 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -1,10 +1,11 @@
-resolver: nightly-2018-09-03
+resolver: lts-21.25
 packages:
 - '.'
 extra-deps:
-- hspec-discover-2.5.6
-- hspec-core-2.5.6
-- hspec-2.5.6
-- QuickCheck-2.12
+- hspec-discover-2.11.7
+- hspec-core-2.11.7
+- hspec-2.11.7
+- QuickCheck-2.14.3
+allow-newer: true
 flags: {}
 extra-package-dbs: []
diff --git a/stack.yaml.lock b/stack.yaml.lock
new file mode 100644
index 0000000..578b1d5
--- /dev/null
+++ b/stack.yaml.lock
@@ -0,0 +1,40 @@
+# This file was autogenerated by Stack.
+# You should not edit this file by hand.
+# For more information, please see the documentation at:
+#   https://docs.haskellstack.org/en/stable/lock_files
+
+packages:
+- completed:
+    hackage: hspec-discover-2.11.7@sha256:6307eb16d308258a99a242025df50217d835ba0a3f205b1202a100a175877b38,2169
+    pantry-tree:
+      sha256: 141b4987d519ad1ca1114737f510f20adc2456bf44c040f41a63792f47d009eb
+      size: 829
+  original:
+    hackage: hspec-discover-2.11.7
+- completed:
+    hackage: hspec-core-2.11.7@sha256:90d8873356d7e15f843bc523360e206e8e356ff6b82a1fa4b3889dc31d073ea1,6814
+    pantry-tree:
+      sha256: 8db12c1f6965d9f0898f04d7b5f1d77682c8ab2d5c394c4a431229df7c4acb14
+      size: 6231
+  original:
+    hackage: hspec-core-2.11.7
+- completed:
+    hackage: hspec-2.11.7@sha256:2869580a2a29e7beb6268ea3dc561583f4ae229ed1f47fb1c92e8c09ce35acec,1763
+    pantry-tree:
+      sha256: f241a7710ebee50583f9aebff61aaf4d36619ba7d6538b39b80ee4695c70552e
+      size: 584
+  original:
+    hackage: hspec-2.11.7
+- completed:
+    hackage: QuickCheck-2.14.3@sha256:f03d2f404d5ba465453d0fbc1944832789a759fe7c4f9bf8616bc1378a02fde4,7847
+    pantry-tree:
+      sha256: e920d61bcc10ef2aee0cc3e2a216d05b8491564ad27c1c5e2fbbb221b00b7f8e
+      size: 2315
+  original:
+    hackage: QuickCheck-2.14.3
+snapshots:
+- completed:
+    sha256: a81fb3877c4f9031e1325eb3935122e608d80715dc16b586eb11ddbff8671ecd
+    size: 640086
+    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/21/25.yaml
+  original: lts-21.25