diff --git a/src/wp-includes/rest-api/class-wp-rest-request.php b/src/wp-includes/rest-api/class-wp-rest-request.php
index d9c24e0658..73548d4f38 100644
--- a/src/wp-includes/rest-api/class-wp-rest-request.php
+++ b/src/wp-includes/rest-api/class-wp-rest-request.php
@@ -408,7 +408,7 @@ class WP_REST_Request implements ArrayAccess {
 		$order = $this->get_parameter_order();
 
 		foreach ( $order as $type ) {
-			if ( array_key_exists( $key, $this->params[ $type ] ) ) {
+			if ( is_array( $this->params[ $type ] ) && array_key_exists( $key, $this->params[ $type ] ) ) {
 				return true;
 			}
 		}
@@ -433,7 +433,7 @@ class WP_REST_Request implements ArrayAccess {
 		$found_key = false;
 
 		foreach ( $order as $type ) {
-			if ( 'defaults' !== $type && array_key_exists( $key, $this->params[ $type ] ) ) {
+			if ( 'defaults' !== $type && is_array( $this->params[ $type ] ) && array_key_exists( $key, $this->params[ $type ] ) ) {
 				$this->params[ $type ][ $key ] = $value;
 				$found_key                     = true;
 			}
diff --git a/tests/phpunit/tests/rest-api/rest-request.php b/tests/phpunit/tests/rest-api/rest-request.php
index 9cbb0f3994..2a78db4660 100644
--- a/tests/phpunit/tests/rest-api/rest-request.php
+++ b/tests/phpunit/tests/rest-api/rest-request.php
@@ -765,4 +765,18 @@ class Tests_REST_Request extends WP_UnitTestCase {
 		$this->assertEquals( array( 'param' => 'new_value' ), $request->get_json_params() );
 		$this->assertEquals( array( 'param' => 'new_value' ), $request->get_query_params() );
 	}
+
+	/**
+	 * @ticket 50786
+	 */
+	public function test_set_param_with_invalid_json() {
+		$request = new WP_REST_Request();
+		$request->add_header( 'content-type', 'application/json' );
+		$request->set_method( 'POST' );
+		$request->set_body( '' );
+		$request->set_param( 'param', 'value' );
+
+		$this->assertTrue( $request->has_param( 'param' ) );
+		$this->assertEquals( 'value', $request->get_param( 'param' ) );
+	}
 }