diff --git a/src/@dseries/firstobservedperiod.m b/src/@dseries/firstobservedperiod.m
index 1f5b287af51a6b60dda6b2670772b2d1d562b1da..fa5d6a29476f12285604a51a0843cc64f4bd8e3c 100644
--- a/src/@dseries/firstobservedperiod.m
+++ b/src/@dseries/firstobservedperiod.m
@@ -27,7 +27,7 @@ function d = firstobservedperiod(o) % --*-- Unitary tests --*--
 
 b = ~isnan(o);
 c = find(prod(b, 2));
-d = o.firstdate+(c(1)-1);
+d = firstdate(o)+(c(1)-1);
 
 %@test:1
 %$ try