diff --git a/composer.json b/composer.json
index ac22899..3c0ea63 100644
--- a/composer.json
+++ b/composer.json
@@ -16,8 +16,9 @@
     },
     "require-dev": {
         "amphp/php-cs-fixer-config": "dev-master",
-        "amphp/phpunit-util": "^1",
-        "phpunit/phpunit": "^6"
+        "amphp/phpunit-util": "^1.1",
+        "phpunit/phpunit": "^9 | ^8 | ^7",
+        "vimeo/psalm": "^3.11@dev"
     },
     "autoload": {
         "psr-4": {
@@ -29,11 +30,6 @@
             "Amp\\Sql\\Test\\": "test"
         }
     },
-    "config": {
-        "platform": {
-            "php": "7.0.0"
-        }
-    },
     "scripts": {
         "check": [
             "@cs",
diff --git a/psalm.xml b/psalm.xml
new file mode 100644
index 0000000..365ac90
--- /dev/null
+++ b/psalm.xml
@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<psalm
+    errorLevel="2"
+    resolveFromConfigFile="true"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xmlns="https://getpsalm.org/schema/config"
+    xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
+>
+    <projectFiles>
+        <directory name="src"/>
+        <ignoreFiles>
+            <directory name="vendor"/>
+        </ignoreFiles>
+    </projectFiles>
+</psalm>
diff --git a/src/ConnectionConfig.php b/src/ConnectionConfig.php
index f65719e..34b49de 100644
--- a/src/ConnectionConfig.php
+++ b/src/ConnectionConfig.php
@@ -46,7 +46,8 @@ abstract class ConnectionConfig
         }
 
         foreach ($params as $param) {
-            list($key, $value) = \array_map("trim", \explode("=", $param, 2) + [1 => null]);
+            /** @psalm-suppress PossiblyInvalidArgument */
+            [$key, $value] = \array_map("trim", \explode("=", $param, 2) + [1 => null]);
 
             if (isset($keymap[$key])) {
                 $key = $keymap[$key];
diff --git a/src/Executor.php b/src/Executor.php
index 8b8eaae..7230e7f 100644
--- a/src/Executor.php
+++ b/src/Executor.php
@@ -9,7 +9,7 @@ interface Executor extends TransientResource
     /**
      * @param string $sql SQL query to execute.
      *
-     * @return Promise<CommandResult|ResultSet>
+     * @return Promise<Result>
      *
      * @throws FailureException If the operation fails due to unexpected condition.
      * @throws ConnectionException If the connection to the database is lost.
@@ -32,7 +32,7 @@ interface Executor extends TransientResource
      * @param string $sql SQL query to prepare and execute.
      * @param mixed[] $params Query parameters.
      *
-     * @return Promise<CommandResult|ResultSet>
+     * @return Promise<Result>
      *
      * @throws FailureException If the operation fails due to unexpected condition.
      * @throws ConnectionException If the connection to the database is lost.
@@ -43,5 +43,5 @@ interface Executor extends TransientResource
     /**
      * Closes the executor. No further queries may be performed.
      */
-    public function close();
+    public function close(): void;
 }
diff --git a/src/QueryError.php b/src/QueryError.php
index 9026530..7d9fdc9 100644
--- a/src/QueryError.php
+++ b/src/QueryError.php
@@ -4,6 +4,7 @@ namespace Amp\Sql;
 
 class QueryError extends \Error
 {
+    /** @var string */
     protected $query = "";
 
     public function __construct(string $message, string $query = "", \Throwable $previous = null)
diff --git a/src/Statement.php b/src/Statement.php
index b29a795..a0a7ae0 100644
--- a/src/Statement.php
+++ b/src/Statement.php
@@ -9,7 +9,7 @@ interface Statement extends TransientResource
     /**
      * @param mixed[] $params
      *
-     * @return Promise<CommandResult|ResultSet>
+     * @return Promise<Result>
      */
     public function execute(array $params = []): Promise;
 
diff --git a/src/Transaction.php b/src/Transaction.php
index decd2d7..79e32cd 100644
--- a/src/Transaction.php
+++ b/src/Transaction.php
@@ -24,7 +24,7 @@ interface Transaction extends Executor
     /**
      * Commits the transaction and makes it inactive.
      *
-     * @return Promise<CommandResult>
+     * @return Promise<Result>
      *
      * @throws TransactionError If the transaction has been committed or rolled back.
      */
@@ -33,7 +33,7 @@ interface Transaction extends Executor
     /**
      * Rolls back the transaction and makes it inactive.
      *
-     * @return Promise<CommandResult>
+     * @return Promise<Result>
      *
      * @throws TransactionError If the transaction has been committed or rolled back.
      */
@@ -44,7 +44,7 @@ interface Transaction extends Executor
      *
      * @param string $identifier Savepoint identifier.
      *
-     * @return Promise<CommandResult>
+     * @return Promise<Result>
      *
      * @throws TransactionError If the transaction has been committed or rolled back.
      */
@@ -55,7 +55,7 @@ interface Transaction extends Executor
      *
      * @param string $identifier Savepoint identifier.
      *
-     * @return Promise<CommandResult>
+     * @return Promise<Result>
      *
      * @throws TransactionError If the transaction has been committed or rolled back.
      */
@@ -66,7 +66,7 @@ interface Transaction extends Executor
      *
      * @param string $identifier Savepoint identifier.
      *
-     * @return Promise<CommandResult>
+     * @return Promise<Result>
      *
      * @throws TransactionError If the transaction has been committed or rolled back.
      */
diff --git a/test/QueryErrorTest.php b/test/QueryErrorTest.php
index 26b40e2..1e19f2d 100644
--- a/test/QueryErrorTest.php
+++ b/test/QueryErrorTest.php
@@ -2,10 +2,10 @@
 
 namespace Amp\Sql\Test;
 
-use Amp\PHPUnit\TestCase;
+use Amp\PHPUnit\AsyncTestCase;
 use Amp\Sql\QueryError;
 
-class QueryErrorTest extends TestCase
+class QueryErrorTest extends AsyncTestCase
 {
     /**
      * @test