Question about -f formatting operator

Why would:

"{0:D4} UNIT HIT ON ENTERPRISE AT SECTOR {1:D1}-{2:D1}   ({3:D4} LEFT)" -f $H, $K[$I-1][1-1], $K[$I-1][2-1], $S

Throw:

Error formatting a string: Format specifier was invalid..
At line:1 char:75+ "{0:D4} UNIT HIT ON ENTERPRISE AT SECTOR {1:D1}-{2:D1}   ({3:D4} LEFT)" -f <<<<  $H, $K[$I-1][1-1], $K[$I-1][2-1], $S+ CategoryInfo          : InvalidOperation: (System.Object[]:Object[]) [], RuntimeException+ FullyQualifiedErrorId : FormatError

But:

$H; $K[$I-1][1-1]; $K[$I-1][2-1]; $S
31
7
7
469
$f1=31;$f2=7;$f3=7;$f4=469"{0:D4} UNIT HIT ON ENTERPRISE AT SECTOR {1:D1}-{2:D1}   ({3:D4} LEFT)" -f $f1,$f2,$f3,$f4
0031 UNIT HIT ON ENTERPRISE AT SECTOR 7-7   (0469 LEFT)

Doesn't?