diff --git a/Packages/vChewing_CandidateWindow/Sources/CandidateWindow/CtlCandidate.swift b/Packages/vChewing_CandidateWindow/Sources/CandidateWindow/CtlCandidate.swift
index a84ef6d7..6b9a90d1 100644
--- a/Packages/vChewing_CandidateWindow/Sources/CandidateWindow/CtlCandidate.swift
+++ b/Packages/vChewing_CandidateWindow/Sources/CandidateWindow/CtlCandidate.swift
@@ -109,7 +109,7 @@ open class CtlCandidate: NSWindowController, CtlCandidateProtocol {
     false
   }
 
-  open func candidateIndexAtKeyLabelIndex(_: Int) -> Int {
+  open func candidateIndexAtKeyLabelIndex(_: Int) -> Int? {
     Int.max
   }
 
diff --git a/Packages/vChewing_Shared/Sources/Shared/Protocols/CtlCandidateProtocol.swift b/Packages/vChewing_Shared/Sources/Shared/Protocols/CtlCandidateProtocol.swift
index d891a44e..07317c78 100644
--- a/Packages/vChewing_Shared/Sources/Shared/Protocols/CtlCandidateProtocol.swift
+++ b/Packages/vChewing_Shared/Sources/Shared/Protocols/CtlCandidateProtocol.swift
@@ -42,7 +42,7 @@ public protocol CtlCandidateProtocol {
   func showPreviousLine() -> Bool
   func highlightNextCandidate() -> Bool
   func highlightPreviousCandidate() -> Bool
-  func candidateIndexAtKeyLabelIndex(_: Int) -> Int
+  func candidateIndexAtKeyLabelIndex(_: Int) -> Int?
   func set(windowTopLeftPoint: NSPoint, bottomOutOfScreenAdjustmentHeight height: Double, useGCD: Bool)
 }
 
diff --git a/Source/Modules/InputHandler_HandleCandidate.swift b/Source/Modules/InputHandler_HandleCandidate.swift
index 65005013..3043ee12 100644
--- a/Source/Modules/InputHandler_HandleCandidate.swift
+++ b/Source/Modules/InputHandler_HandleCandidate.swift
@@ -141,8 +141,7 @@ extension InputHandler {
     }
 
     if index != NSNotFound {
-      let candidateIndex = ctlCandidate.candidateIndexAtKeyLabelIndex(index)
-      if candidateIndex != -114_514 {
+      if let candidateIndex = ctlCandidate.candidateIndexAtKeyLabelIndex(index) {
         delegate.candidateSelectionCalledByInputHandler(at: candidateIndex)
         return true
       }
@@ -185,13 +184,10 @@ extension InputHandler {
       }
 
       if shouldAutoSelectCandidate {
-        let candidateIndex = ctlCandidate.candidateIndexAtKeyLabelIndex(0)
-        if candidateIndex != -114_514 {
-          delegate.candidateSelectionCalledByInputHandler(at: candidateIndex)
-          delegate.switchState(IMEState.ofAbortion())
-          return handleInput(event: input)
-        }
-        return true
+        guard let candidateIndex = ctlCandidate.candidateIndexAtKeyLabelIndex(0) else { return true }
+        delegate.candidateSelectionCalledByInputHandler(at: candidateIndex)
+        delegate.switchState(IMEState.ofAbortion())
+        return handleInput(event: input)
       }
     }
 
diff --git a/Source/Modules/UIModules/CandidateUI/IMKCandidatesImpl.swift b/Source/Modules/UIModules/CandidateUI/IMKCandidatesImpl.swift
index 26462f1a..a1a751eb 100644
--- a/Source/Modules/UIModules/CandidateUI/IMKCandidatesImpl.swift
+++ b/Source/Modules/UIModules/CandidateUI/IMKCandidatesImpl.swift
@@ -141,7 +141,7 @@ public class CtlCandidateIMK: IMKCandidates, CtlCandidateProtocol {
   }
 
   // IMK 選字窗目前無法實作該函式。威注音 IMK 選字窗目前也不需要使用該函式。
-  public func candidateIndexAtKeyLabelIndex(_: Int) -> Int { 0 }
+  public func candidateIndexAtKeyLabelIndex(_: Int) -> Int? { 0 }
 
   public var highlightedIndex: Int {
     get { selectedCandidate() }