title: Operator '+' cannot be applied to types '{}' and 'number' Ask
body: <p>I'm using TypeScript 2.4, sbt, and underscore.
  When I use.</p>
  
  <pre><code>this.userData.summaryMonth = _.reduce(this.userData.monthValues, (m, n) =&gt; m+n, 0);
  </code></pre>
  
  <p>My interface </p>
  
  <pre><code>export interface IUserPlan {
      id: number,
      login: string,
      fullName: string,
      role: IUserPlanRole,
      month?: string[]
      monthValues?: number[],
      summaryDay?: 0,
      summaryMonth?: number,
      workingDays?: number,
      nonWorkingDays?: number,
      filterType: string
  }
  </code></pre>
  
  <p>I'm getting an error:</p>
  
  <pre><code>[info] TypeScript compiling on 1 source(s)
  [error] /developer/projects/------/app/assets/admin/statistics/statistics.controller.ts:220: Operator '+' cannot be applied to types 'number' and '{}'.
  [error] this.userData.summaryMonth = _.reduce(this.userData.monthValues, (m, n) =&gt; m+n, 0);                                                                                                        
          ^
  [error] one error found
  [error] (bo/web-assets:typescript) com.typesafe.sbt.web.CompileProblemsException
  </code></pre>