diff --git a/CHANGELOG.md b/CHANGELOG.md
index bf69b55..24b00db 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+#### 4.2.3
+
+* Rollback to version 4.2.1 (https://github.com/google/process.dart/issues/64)
+
 #### 4.2.2
 
 * Mark `stderrEncoding` and `stdoutEncoding` parameters as nullable.
diff --git a/lib/src/interface/local_process_manager.dart b/lib/src/interface/local_process_manager.dart
index 7de97a3..72e513a 100644
--- a/lib/src/interface/local_process_manager.dart
+++ b/lib/src/interface/local_process_manager.dart
@@ -67,8 +67,8 @@
     Map<String, String>? environment,
     bool includeParentEnvironment = true,
     bool runInShell = false,
-    Encoding? stdoutEncoding = systemEncoding,
-    Encoding? stderrEncoding = systemEncoding,
+    Encoding stdoutEncoding = systemEncoding,
+    Encoding stderrEncoding = systemEncoding,
   }) {
     try {
       return Process.run(
@@ -98,8 +98,8 @@
     Map<String, String>? environment,
     bool includeParentEnvironment = true,
     bool runInShell = false,
-    Encoding? stdoutEncoding = systemEncoding,
-    Encoding? stderrEncoding = systemEncoding,
+    Encoding stdoutEncoding = systemEncoding,
+    Encoding stderrEncoding = systemEncoding,
   }) {
     try {
       return Process.runSync(
diff --git a/lib/src/interface/process_manager.dart b/lib/src/interface/process_manager.dart
index d99a874..8862a14 100644
--- a/lib/src/interface/process_manager.dart
+++ b/lib/src/interface/process_manager.dart
@@ -141,9 +141,8 @@
     Map<String, String>? environment,
     bool includeParentEnvironment = true,
     bool runInShell = false,
-    // TODO(#64): Remove the `covariant` keyword.
-    covariant Encoding? stdoutEncoding = systemEncoding,
-    covariant Encoding? stderrEncoding = systemEncoding,
+    Encoding stdoutEncoding = systemEncoding,
+    Encoding stderrEncoding = systemEncoding,
   });
 
   /// Starts a process and runs it to completion. This is a synchronous
@@ -159,9 +158,8 @@
     Map<String, String>? environment,
     bool includeParentEnvironment = true,
     bool runInShell = false,
-    // TODO(#64): Remove the `covariant` keyword.
-    covariant Encoding? stdoutEncoding = systemEncoding,
-    covariant Encoding? stderrEncoding = systemEncoding,
+    Encoding stdoutEncoding = systemEncoding,
+    Encoding stderrEncoding = systemEncoding,
   });
 
   /// Returns `true` if the [executable] exists and if it can be executed.
diff --git a/pubspec.yaml b/pubspec.yaml
index 7cb3e93..6458b06 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: process
-version: 4.2.2
+version: 4.2.3
 description: A pluggable, mockable process invocation abstraction for Dart.
 homepage: https://github.com/google/process.dart