diff --git a/allan_cov.m b/allan_cov.m
index f35537b..75da6ef 100644
--- a/allan_cov.m
+++ b/allan_cov.m
@@ -328,7 +328,7 @@ if isfield(data,'rate') && data.rate > 0 % if data rate was given
 		fd2=diff(fa2);
 		% calculate two-sample variance for this tau
 		M=length(fa);
-		sm(k)=sqrt(0.5/(M-1)*(real(sum(fd.*fd2))));
+		sm(k)=sqrt(0.5/(M-1)*abs(real(sum(fd.*fd2))));
 
 		% estimate error bars
 		sme(k)=sm(k)/sqrt(M+1);