Cannot read property 'setTexture' of undefined

Hello,

I’m trying to change a sprite to another and I’m getting this weird error message: Cannot read property ‘setTexture’ of undefined

Here is the line of code triggering the error:
this.enemy1_display.setTexture(this.getEnemyString(displayed[0]));

getEnemyString() returns a string which correlates with a sprite that I’ve already preloaded, and enemy1_display is a sprite that I’ve created like this:
this.enemy1_display = this.physics.add.sprite(830, 90, "enemies1");

This is strange because I’ve done a number of similar things already in the same file. A bit above, I use this line of code and it works perfectly
this.player.setTexture('ship1')

I’ve also already used the ‘getEnemyString()’ method in a very similar way here:
var enemy = this.enemies1.create((x + 2) * (550 / 7) - 10, ((y + 2) * 50) - 35, this.getEnemyString(displayed[0]));

That means

this.enemy1_display.setTexture(this.getEnemyString(displayed[0]));

is

(undefined).setTexture(this.getEnemyString(displayed[0]));