BelongsTo Relationship returns null

I am building a table that has either Clients or Leads, so I have two three tables:

• Activities
• Leads
• Clients

I try to get either the client or the lead name by using relationship, in the model:

if ($this->lead_type== 'Lead') { return $this->belongsTo(Lead::class, 'lead') } else { return $this->belongTo(Client::class, 'lead') } If I try to return either one of the twos, I get the result, but when I try to put the if condition, only the second on query returns a value. The first one returns null.

I am using Laravel Query Builder by Spatie.