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 3257194e547d2..96e9468175eda 100644
--- a/src/wp-includes/rest-api/class-wp-rest-request.php
+++ b/src/wp-includes/rest-api/class-wp-rest-request.php
@@ -458,13 +458,23 @@ public function set_param( $key, $value ) {
 		$found_key = false;
 
 		foreach ( $order as $type ) {
-			if ( 'defaults' !== $type && is_array( $this->params[ $type ] ) && array_key_exists( $key, $this->params[ $type ] ) ) {
-				$this->params[ $type ][ $key ] = $value;
-				$found_key                     = true;
+			if ( 'defaults' !== $type && isset( $this->params[ $type ] ) ) {
+
+				if ( ! is_array( $this->params[ $type ] ) ) {
+					$this->params[ $type ] = array();
+				}
+
+				if ( array_key_exists( $key, $this->params[ $type ] ) ) {
+					$this->params[ $type ][ $key ] = $value;
+					$found_key                     = true;
+				}
 			}
 		}
 
 		if ( ! $found_key ) {
+			if ( ! isset( $this->params[ $order[0] ] ) || ! is_array( $this->params[ $order[0] ] ) ) {
+				$this->params[ $order[0] ] = array();
+			}
 			$this->params[ $order[0] ][ $key ] = $value;
 		}
 	}
@@ -821,7 +831,7 @@ public function sanitize_params() {
 		$invalid_details = array();
 
 		foreach ( $order as $type ) {
-			if ( empty( $this->params[ $type ] ) ) {
+			if ( empty( $this->params[ $type ] ) || ! is_array( $this->params[ $type ] ) ) {
 				continue;
 			}
 
diff --git a/tests/phpunit/tests/rest-api/rest-request.php b/tests/phpunit/tests/rest-api/rest-request.php
index 1067739ed2469..fcc6089ecf5b4 100644
--- a/tests/phpunit/tests/rest-api/rest-request.php
+++ b/tests/phpunit/tests/rest-api/rest-request.php
@@ -1082,6 +1082,92 @@ public function test_route_level_validate_callback_is_not_executed_if_parameter_
 		$this->assertSame( 'rest_invalid_param', $valid->get_error_code() );
 	}
 
+	/**
+	 * @ticket 61061
+	 */
+	public function test_set_param_with_non_array_parameter_types() {
+		$request = new WP_REST_Request();
+		$request->add_header( 'Content-Type', 'application/json' );
+		$request->set_method( 'POST' );
+		$request->set_body( '"invalid json string"' );
+
+		$request->set_param( 'test_param', 'test_value' );
+
+		$this->assertTrue( $request->has_param( 'test_param' ) );
+		$this->assertSame( 'test_value', $request->get_param( 'test_param' ) );
+	}
+
+	/**
+	 * @ticket 61061
+	 */
+	public function test_set_param_with_malformed_json_from_curl_request() {
+		$request = new WP_REST_Request();
+		$request->add_header( 'Content-Type', 'application/json' );
+		$request->set_method( 'POST' );
+		$request->set_body( '"+response.write(document.domain)+"' );
+
+		$request->set_param( 'test_param', 'test_value' );
+
+		$this->assertTrue( $request->has_param( 'test_param' ) );
+		$this->assertSame( 'test_value', $request->get_param( 'test_param' ) );
+	}
+
+	/**
+	 * @ticket 61061
+	 */
+	public function test_sanitize_and_validate_params_with_non_array_parameter_types() {
+		$request = new WP_REST_Request();
+		$request->add_header( 'Content-Type', 'application/json' );
+		$request->set_method( 'POST' );
+		$request->set_body( '"invalid json string"' );
+
+		$request->set_attributes(
+			array(
+				'args' => array(
+					'someinteger'    => array(
+						'sanitize_callback' => 'absint',
+					),
+					'required_param' => array(
+						'required' => true,
+					),
+				),
+			)
+		);
+
+		$sanitize_result = $request->sanitize_params();
+		$this->assertTrue( $sanitize_result );
+
+		$validation_result = $request->has_valid_params();
+		$this->assertWPError( $validation_result );
+		$this->assertSame( 'rest_missing_callback_param', $validation_result->get_error_code() );
+	}
+
+	/**
+	 * @ticket 61061
+	 */
+	public function test_scalar_request_body_should_not_throw_fatal_errors() {
+
+		$user_id = self::factory()->user->create(
+			array(
+				'role' => 'editor',
+			)
+		);
+
+		wp_set_current_user( $user_id );
+
+		$request = new WP_REST_Request( 'POST', sprintf( '/wp/v2/users/%d', $user_id ) );
+		$request->set_header( 'Content-Type', 'application/json' );
+		$request->set_body( '"+response.write(document.domain)+"' );
+
+		try {
+			$response = rest_get_server()->dispatch( $request );
+			$this->assertNotNull( $response, 'Response should not be null' );
+			$this->assertInstanceOf( 'WP_REST_Response', $response, 'Response should be a WP_REST_Response object' );
+		} catch ( Exception $e ) {
+			$this->fail( 'Fatal error thrown when parsing scalar request body.' );
+		}
+	}
+
 	/**
 	 * Tests that WP_REST_Request::is_method() correctly detects the request method,
 	 * regardless of case sensitivity.