diff --git a/rip44d b/rip44d
index 5e56fd1..3dd0c46 100755
--- a/rip44d
+++ b/rip44d
@@ -152,7 +152,7 @@ sub mask2prefix($)
 	return -1 if ($bits !~ /^(1*)(0*)$/);
 	
 	# The amount of 1's in the beginning is the prefix length.
-	return length($bits);
+	return length($1);
 }
 
 # delete a route from the kernel's table