diff --git a/CHANGELOG-Japanese.md b/CHANGELOG-Japanese.md
index ab541cea..3a15f502 100644
--- a/CHANGELOG-Japanese.md
+++ b/CHANGELOG-Japanese.md
@@ -16,6 +16,7 @@
 - 無効なAPIキーが指定された場合に、VirusTotalの検索でJSONパースエラーが発生する問題を修正した。(@fukusuket)
 - `sysmon-process-tree`コマンドでプロセス情報が2回出力されることがあるバグを修正した。(#52) (@fukusuket)
 - `timeline-suspicious-processes`が`ParentPGUID`フィールドを正しく出力していなかったので修正した。また、PIDの10進数変換を改善した。(#50) (@fukusuket)
+- 指定された`PGUID`が無効であるか、JSONL タイムラインに存在しない場合にエラーが発生する問題を修正した。 (#53) (@fukusuket)
 
 ## 2.0.0 [2022/08/03] - [SANS DFIR Summit 2023 Release](https://www.sans.org/cyber-security-training-events/digital-forensics-summit-2023/)
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 22a50e47..bdb7e74c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@
 - Fixed a JSON parsing error in VirusTotal lookups when an invalid API key was specified. (@fukusuket)
 - Fixed a bug in `sysmon-process-tree` in which process information would sometimes be outputted twice. (#52) (@fukusuket)
 - `timeline-suspicious-processes` was not correctly outputting `ParentPGUID` field. Improved PID decimal conversion. (#50) (@fukusuket)
+- Fixed an error when the specified `PGUID` was invalid or does not exist in the JSONL timeline. (#53) (@fukusuket)
 
 ## 2.0.0 [2022/08/03] - [SANS DFIR Summit 2023 Release](https://www.sans.org/cyber-security-training-events/digital-forensics-summit-2023/)
 
diff --git a/src/takajo.nim b/src/takajo.nim
index ed4fdaf7..091708e5 100644
--- a/src/takajo.nim
+++ b/src/takajo.nim
@@ -2,6 +2,7 @@ import algorithm
 import cligen
 import json
 import puppy
+import re
 import sets
 import sequtils
 import strformat
diff --git a/src/takajopkg/sysmonProcessTree.nim b/src/takajopkg/sysmonProcessTree.nim
index 602b8f9c..409f127d 100644
--- a/src/takajopkg/sysmonProcessTree.nim
+++ b/src/takajopkg/sysmonProcessTree.nim
@@ -62,6 +62,10 @@ proc moveProcessObjectToChild(mvSourceProcess: processObject,
             moveProcessObjectToChild(mvSourceProcess, child,
                     outputProcess.children[idx])
 
+proc isGUID(processGuid: string): bool =
+    let guidRegex = re(r"^[0-9A-F]{8}-[0-9A-F]{4}-[0-9A-F]{4}-[0-9A-F]{4}-[0-9A-F]{12}$")
+    return processGuid.find(guidRegex) != -1
+
 proc sysmonProcessTree(output: string = "", processGuid: string,
         quiet: bool = false, timeline: string) =
     ## Procedure for displaying Sysmon's process tree
@@ -77,6 +81,10 @@ proc sysmonProcessTree(output: string = "", processGuid: string,
     if not isJsonConvertible(timeline):
         quit(1)
 
+    if not isGUID(processGuid):
+        echo "The format of the Process GUID specified with the -p option is invalid. Please specify a valid Process GUID."
+        quit(1)
+
     echo ""
     echo "Running the Process Tree module"
     echo ""
@@ -210,9 +218,6 @@ proc sysmonProcessTree(output: string = "", processGuid: string,
                     processGUID: eventProcessGUID,
                     parentProcessGUID: foundProcessTable["ParentPGUID"])
             let key = timeStamp & "-" & process.processID
-            # if addedProcess.contains(key):
-            #     continue
-
             if not passGuid.contains(eventProcessGUID):
                 passGuid.incl(eventProcessGUID)
             if not passGuid.contains(process.parentProcessGUID):
@@ -227,6 +232,12 @@ proc sysmonProcessTree(output: string = "", processGuid: string,
             parentProcessGUIDTable[process.parentProcessGUID] = process.processGUID
             parents_exist = true
             parents_key = process.processGUID
+
+    if processGuid notin stockedProcessObjectTable:
+      echo "The process was not found."
+      echo ""
+      return
+
     var outputStrSeq: seq[string] = @[]
     var outputProcessObjectTable = stockedProcessObjectTable
 
@@ -247,8 +258,8 @@ proc sysmonProcessTree(output: string = "", processGuid: string,
 
 
     # Display process tree for the specified process root
-    let root_multi_child = outputProcessObjectTable[parents_key].children.len() > 1
     if parents_key != "":
+        let root_multi_child = outputProcessObjectTable[parents_key].children.len() > 1
         outputStrSeq = concat(outputStrSeq, printIndentedProcessTree(
                 outputProcessObjectTable[parents_key], need_sameStair = @[
                         root_multi_child], parentsStair = false
@@ -256,7 +267,7 @@ proc sysmonProcessTree(output: string = "", processGuid: string,
     elif outputProcessObjectTable.hasKey(processGuid):
         outputStrSeq = concat(outputStrSeq, printIndentedProcessTree(
                 outputProcessObjectTable[processGuid], need_sameStair = @[
-                        root_multi_child], parentsStair = false))
+                        false], parentsStair = false))
 
     if output != "":
         let f = open(output, fmWrite)