diff --git a/luasrc/controller/dockerman.lua b/luasrc/controller/dockerman.lua
index 7467bc0..b11acf7 100644
--- a/luasrc/controller/dockerman.lua
+++ b/luasrc/controller/dockerman.lua
@@ -75,10 +75,17 @@ local calculate_cpu_percent = function(d)
 
 	local cpu_count = tonumber(d["cpu_stats"]["online_cpus"])
 	local cpu_percent = 0.0
-	local cpu_delta = tonumber(d["cpu_stats"]["cpu_usage"]["total_usage"]) - tonumber(d["precpu_stats"]["cpu_usage"]["total_usage"])
-	local system_delta = tonumber(d["cpu_stats"]["system_cpu_usage"]) - tonumber(d["precpu_stats"]["system_cpu_usage"])
-	if system_delta > 0.0 then
-		cpu_percent = string.format("%.2f", cpu_delta / system_delta * 100.0 * cpu_count)
+	local cpu_total_usage_current = tonumber(d["cpu_stats"]["cpu_usage"]["total_usage"])
+	local cpu_total_usage_previous = tonumber(d["precpu_stats"]["cpu_usage"]["total_usage"])
+	local system_cpu_usage_current = tonumber(d["cpu_stats"]["system_cpu_usage"])
+	local system_cpu_usage_previous = tonumber(d["precpu_stats"]["system_cpu_usage"])
+
+	if cpu_total_usage_current and cpu_total_usage_previous and system_cpu_usage_current and system_cpu_usage_previous then
+		local cpu_delta = cpu_total_usage_current - cpu_total_usage_previous
+		local system_delta = system_cpu_usage_current - system_cpu_usage_previous
+		if system_delta > 0.0 then
+			cpu_percent = string.format("%.2f", cpu_delta / system_delta * 100.0 * cpu_count)
+		end
 	end
 
 	return cpu_percent
@@ -89,10 +96,11 @@ local get_memory = function(d)
 		return
 	end
 
-	local limit =tonumber(d["memory_stats"]["limit"])
-	local usage = tonumber(d["memory_stats"]["usage"]) - tonumber(d["memory_stats"]["stats"]["total_cache"])
+	local limit = tonumber(d["memory_stats"]["limit"])
+	local usage = tonumber(d["memory_stats"]["usage"]) or 0
+	local total_cache = tonumber(d["memory_stats"]["stats"]["total_cache"]) or 0
 
-	return usage, limit
+	return usage - total_cache, limit
 end
 
 local get_rx_tx = function(d)