Unexpected display of "non-data"

I have been making minor adjustments to a project that I haven't touched for a year or so. This involved printing an int and some floats to LCD. There is nothing special here

float flowRate;
float kW, kWh;
float flowMilliLitres;   //QUANTITY
int totalLitres;

and the print srtn is

void LCDdata(){
  lcd.setCursor(49,0);
  lcd.print(InTemp);
  lcd.setCursor(49,1);
  lcd.print (OutTemp); 
  lcd.setCursor(49,2);
  lcd.print(flowRate,1);
  lcd.setCursor(49,3);
  lcd.print(totalLitres);  
  lcd.setCursor(49,4);\
  lcd.print (kW);  
  lcd.print(" ");//suppress trailing fig > 9.99
  lcd.setCursor(49,5);
  lcd.print (kWh);

When I uploaded the programme, no sensors were connected. The flow was displayed as "inf", a total was zero, which is understandable, and the two others are displayed as "nan".

I don't recall what might have been displayed before, but I didn't expect any of this. I can understand "inf" and shouldn't be that surprised, but what would "nan" mean? I am using R1.56-r2. Is this something new to that version?

Not a Number.

Is this something new to that version?

Not even a skosh.

Not a number...... I should have known........