diff --git a/R/R/metrics.r b/R/R/metrics.r
index 3d8ad57..b957542 100644
--- a/R/R/metrics.r
+++ b/R/R/metrics.r
@@ -166,6 +166,10 @@ apk <- function(k, actual, predicted)
 {
     score <- 0.0
     cnt <- 0.0
+    if(length(actual) == 0)
+    {
+    	return(0)
+    }
     for (i in 1:min(k,length(predicted)))
     {
         if (predicted[i] %in% actual && !(predicted[i] %in% predicted[0:(i-1)]))