json_encode(): Invalid UTF-8 sequence in argument in laravel

I got following result when i do dd($somevariable) in laravel

array:12 [
  0 => {#223
    +"jobinspt_post_questions_id": 117
    +"category": "Fascia & Gutter"
    +"question": "The F&G is complete"
    +"subquestion": null
    +"passed": null
    +"rectification": null
    +"catpassed": "N"
  }
  1 => {#224
    +"jobinspt_post_questions_id": 118
    +"category": "Fascia & Gutter"
    +"question": "Temporary props are in use"
    +"subquestion": null
    +"passed": null
    +"rectification": null
    +"catpassed": "N"
  }
  2 => {#225
    +"jobinspt_post_questions_id": 119
    +"category": "Fascia & Gutter"
    +"question": "The F&G is straight, level and undamaged"
    +"subquestion": null
    +"passed": null
    +"rectification": null
    +"catpassed": "N"
  }
  3 => {#226
    +"jobinspt_post_questions_id": 120
    +"category": "Fascia & Gutter"
    +"question": "F&G at correct height and distance from frame"
    +"subquestion": null
    +"passed": null
    +"rectification": null
    +"catpassed": "N"
  }
  4 => {#227
    +"jobinspt_post_questions_id": 121
    +"category": "Fascia & Gutter"
    +"question": "Dutch gables are cut neatly and level"
    +"subquestion": null
    +"passed": null
    +"rectification": null
    +"catpassed": "N"
  }
  5 => {#228
    +"jobinspt_post_questions_id": 122
    +"category": "Fascia & Gutter"
    +"question": "All rafter brackets are installed"
    +"subquestion": null
    +"passed": null
    +"rectification": null
    +"catpassed": "N"
  }
  6 => {#229
    +"jobinspt_post_questions_id": 123
    +"category": "Fascia & Gutter"
    +"question": "All barge gutters/ returns are installed"
    +"subquestion": null
    +"passed": null
    +"rectification": null
    +"catpassed": "N"
  }
  7 => {#230
    +"jobinspt_post_questions_id": 124
    +"category": "Fascia & Gutter"
    +"question": "Hips and jacks are propped"
    +"subquestion": null
    +"passed": null
    +"rectification": null
    +"catpassed": "N"
  }
  8 => {#231
    +"jobinspt_post_questions_id": 125
    +"category": "Fascia & Gutter"
    +"question": "Ext & Int corners are installed"
    +"subquestion": null
    +"passed": null
    +"rectification": null
    +"catpassed": "N"
  }
  9 => {#232
    +"jobinspt_post_questions_id": 126
    +"category": "Fascia & Gutter"
    +"question": "Rake downs/ laybacks/drag downs installed"
    +"subquestion": null
    +"passed": null
    +"rectification": null
    +"catpassed": "N"
  }
  10 => {#233
    +"jobinspt_post_questions_id": 127
    +"category": "Fascia & Gutter"
    +"question": b"Rivets – correct colour and number"
    +"subquestion": null
    +"passed": null
    +"rectification": null
    +"catpassed": "N"
  }
  11 => {#234
    +"jobinspt_post_questions_id": 128
    +"category": "Fascia & Gutter"
    +"question": "Downpipes pops or drops installed as per plan"
    +"subquestion": null
    +"passed": null
    +"rectification": null
    +"catpassed": "N"
  }

]

when i encode this array to json i got the following error

json_encode(): Invalid UTF-8 sequence in argument

I dont know where is that invalid utf8 sequence. Please help me to find out what is wrong with this array