Get Error with Predicate when using nbt (only on Server)

Got Error Message Unterminated object in this predicate file:

{
  "condition": "minecraft:entity_properties",
  "entity": "this",
  "predicate": {
    "type": "player",
    "nbt": "{SelectedItem: {id:'minecraft:command_block'}}"
  }
}

I tried something else, such as

...edItem: {id:\"*minecraft:command_block\"*}}"
But I alsways get this error in this line after "{id:".

"nbt": "{SelectedItem: {id:'minecraft:command_block'}}"

Same datapack works fine in singleplayer.