reactjs – React propType无法读取undefined的属性

TypeError: Cannot read property ‘string’ of undefined.

TypeError: Cannot read property ‘func’ of undefined.

有问题的代码位于代码段的底部：

import React from 'react';
import ProjectItem from './ProjectItem';

class Projects extends React.Component {

    deleteProject(title) {
      this.props.onDelete(title);
    }

    render() { 
      let projectItems;

      if (this.props.project) {
        projectItems = this.props.project.map(project => {
            return (
                <ProjectItem key={project.title} project={project} onDelete={this.deleteProject.bind(this)}  />
            )
        });
      }

      return (
        <div className="Projects">
          {projectItems}
        </div>    
      );    
    }

}

Projects.propTypes = {
  projects: React.PropTypes.string,onDelete: React.PropTypes.func
}