diff --git a/doc/100-General/10-Changelog.md b/doc/100-General/10-Changelog.md
index cd1d93e..85f834c 100644
--- a/doc/100-General/10-Changelog.md
+++ b/doc/100-General/10-Changelog.md
@@ -20,6 +20,7 @@ Released closed milestones can be found on [GitHub](https://github.com/Icinga/ic
 * [#621](https://github.com/Icinga/icinga-powershell-framework/pull/621) Fixes `-ThresholdInterval` key detection on newer systems
 * [#645](https://github.com/Icinga/icinga-powershell-framework/pull/645) Fixes error and exception handling while using API-Checks, which now will in most cases always return a proper check-result object and also abort while running into plugin execution errors, in case a server is not reachable by the time sync plugin for example
 * [#646](https://github.com/Icinga/icinga-powershell-framework/pull/646) Fixes REST-Api to allow arguments for check execution with and without leading `-`
+* [#648](https://github.com/Icinga/icinga-powershell-framework/pull/648) Fixes some memory management while using the REST-Api to clear connection objects once they are no longer required
 
 ### Enhancements
 
diff --git a/lib/daemons/RestAPI/daemon/New-IcingaForWindowsRESTApi.psm1 b/lib/daemons/RestAPI/daemon/New-IcingaForWindowsRESTApi.psm1
index b8c0f21..f249e9a 100644
--- a/lib/daemons/RestAPI/daemon/New-IcingaForWindowsRESTApi.psm1
+++ b/lib/daemons/RestAPI/daemon/New-IcingaForWindowsRESTApi.psm1
@@ -87,16 +87,19 @@ function New-IcingaForWindowsRESTApi()
         if (Test-IcingaRESTClientBlacklisted -Client $Connection.Client -ClientList $Global:Icinga.Public.Daemons.RESTApi.ClientBlacklist) {
             Write-IcingaDebugMessage -Message 'A remote client which is trying to connect was blacklisted' -Objects $Connection.Client.Client;
             Close-IcingaTCPConnection -Client $Connection.Client;
+            $Connection = $null;
             continue;
         }
 
         if ((Test-IcingaRESTClientConnection -Connection $Connection) -eq $FALSE) {
+            $Connection = $null;
             continue;
         }
 
         # API not yet ready
         if ($Global:Icinga.Public.Daemons.RESTApi.ApiRequests.Count -eq 0) {
             Close-IcingaTCPConnection -Client $Connection.Client;
+            $Connection = $null;
             continue;
         }
 
@@ -105,6 +108,7 @@ function New-IcingaForWindowsRESTApi()
 
             if ($Global:Icinga.Public.Daemons.RESTApi.ApiRequests.ContainsKey($NextRESTApiThreadId) -eq $FALSE) {
                 Close-IcingaTCPConnection -Client $Connection.Client;
+                $Connection = $null;
                 continue;
             }
 
diff --git a/lib/daemons/RestAPI/threads/New-IcingaForWindowsRESTThread.psm1 b/lib/daemons/RestAPI/threads/New-IcingaForWindowsRESTThread.psm1
index 3e00104..d1656f4 100644
--- a/lib/daemons/RestAPI/threads/New-IcingaForWindowsRESTThread.psm1
+++ b/lib/daemons/RestAPI/threads/New-IcingaForWindowsRESTThread.psm1
@@ -39,6 +39,7 @@ function New-IcingaForWindowsRESTThread()
                         Send-IcingaWebAuthMessage -Connection $Connection;
                         # Close the connection
                         Close-IcingaTCPConnection -Client $Connection.Client;
+                        $Connection = $null;
                         continue;
                     }
 
@@ -56,6 +57,7 @@ function New-IcingaForWindowsRESTThread()
                         Send-IcingaWebAuthMessage -Connection $Connection;
                         # Close the connection
                         Close-IcingaTCPConnection -Client $Connection.Client;
+                        $Connection = $null;
                         continue;
                     }
                 }
@@ -99,6 +101,7 @@ function New-IcingaForWindowsRESTThread()
         # Finally close the clients connection as we are done here and
         # ensure this thread will close by simply leaving the function
         Close-IcingaTCPConnection -Client $Connection.Client;
+        $Connection = $null;
 
         # Force Icinga for Windows Garbage Collection
         Optimize-IcingaForWindowsMemory -ClearErrorStack -SmartGC;
diff --git a/lib/daemons/modules/ApiChecks/Invoke-IcingaApiChecksRESTCall.psm1 b/lib/daemons/modules/ApiChecks/Invoke-IcingaApiChecksRESTCall.psm1
index 0dcbe65..bb92543 100644
--- a/lib/daemons/modules/ApiChecks/Invoke-IcingaApiChecksRESTCall.psm1
+++ b/lib/daemons/modules/ApiChecks/Invoke-IcingaApiChecksRESTCall.psm1
@@ -57,6 +57,26 @@ function Invoke-IcingaApiChecksRESTCall()
             [string]$ExecuteCommand = $Request.RequestArguments.command;
         }
 
+        if ((Test-IcingaFunction -Name $ExecuteCommand) -eq $FALSE) {
+
+            Add-IcingaHashtableItem `
+                -Hashtable $ContentResponse `
+                -Key $ExecuteCommand `
+                -Value @{
+                    'exitcode'    = 3;
+                    'checkresult' = [string]::Format('[UNKNOWN] Icinga plugin not found exception: Command "{0}" is not present on the system{1}{1}The command "{0}" you are trying to execute over the REST-Api endpoint "apichecks" is not available on the system.', $ExecuteCommand, (New-IcingaNewLine));
+                    'perfdata'    = @();
+                } | Out-Null;
+
+            Send-IcingaTCPClientMessage -Message (
+                New-IcingaTCPClientRESTMessage `
+                    -HTTPResponse ($IcingaHTTPEnums.HTTPResponseType.'Not Found') `
+                    -ContentBody $ContentResponse
+            ) -Stream $Connection.Stream;
+
+            return;
+        }
+
         if ((Test-IcingaRESTApiCommand -Command $ExecuteCommand -Endpoint 'apichecks') -eq $FALSE) {
 
             Add-IcingaHashtableItem `