diff --git a/src/Doctrine/Migration.php b/src/Doctrine/Migration.php
index c6b35de..342b66e 100644
--- a/src/Doctrine/Migration.php
+++ b/src/Doctrine/Migration.php
@@ -139,6 +139,10 @@ protected function setColumnNullable(string $table, string $name): void
     protected function setColumnNotNullable(string $table, string $name): void
     {
         $constraintName = sprintf('chk_null_%s_%s', $table, $name);
+        $this->addUnsafeSql(sprintf('ALTER TABLE %s DROP CONSTRAINT IF EXISTS "%s"',
+            $table,
+            $constraintName,
+        ));
         $this->addUnsafeSql(sprintf('ALTER TABLE %s ADD CONSTRAINT "%s" CHECK (%s IS NOT NULL) NOT VALID',
             $table,
             $constraintName,
diff --git a/tests/Database/Doctrine/Migrations/MigrationTest.php b/tests/Database/Doctrine/Migrations/MigrationTest.php
index ae3c191..3c506a4 100644
--- a/tests/Database/Doctrine/Migrations/MigrationTest.php
+++ b/tests/Database/Doctrine/Migrations/MigrationTest.php
@@ -213,6 +213,7 @@ public function testSetColumnNotNullable(): void
         $this->migration->setColumnNotNullable('signer', 'email');
 
         $this->assertSql([
+            'ALTER TABLE signer DROP CONSTRAINT IF EXISTS "chk_null_signer_email"',
             'ALTER TABLE signer ADD CONSTRAINT "chk_null_signer_email" CHECK (email IS NOT NULL) NOT VALID',
             "SET statement_timeout TO '0'",
             'ALTER TABLE signer VALIDATE CONSTRAINT "chk_null_signer_email"',