2nd foot of this hemiepes: SEMITA VIRTUS AGAT

Posting about another sundial inscription that looks fishy...

UMBRA TEGIT LAPSAS PRAESENTIQUE IMMINET HORAE,
DUM LUX, DUM LUCIS SEMITA VIRTUS AGAT

My close translation: “The shadow covers those having slipped by, and it threatens the present hour;
while [there’s] light, [and] while there’s light’s path, let goodness lead.”

(UM-bra te)-(GIT LĀP)-(SĀS PRAE)-(SEN-TĪ)-(QU<E> IM-mi-ne)-(T <H>Ō-RAE)

(H     L     L)   (H    H)       (H     H)         (H      H)   (H                 L   L)    (H            H)

(DUM LŪX) (DUM LŪ)-(CIS) | (SĒ-mi-ta) (VIR-TŪ-sa)-(GAT)

(H        H)       (H       H)    (H)     | (H   L   L)  (H      H!  L)   (H)

Is the U of VIRTUS not long? Everything about it suggests an elegiac couplet, except for the 2nd line's 2nd half's 2nd foot's, 2nd syllable's weight (!). I.e., (VIR-TŪ-sa). I was expecting a dactyl, a light second syllable. Does this type of foot (H H L) even have a name?

Or is this a mistake, either on my part or the person who composed it?

Thank you in advance!