diff --git a/Source/3rdParty/OVMandarin/Composer.hh b/Source/3rdParty/OVMandarin/Composer.hh
index d620b766..fa49864f 100644
--- a/Source/3rdParty/OVMandarin/Composer.hh
+++ b/Source/3rdParty/OVMandarin/Composer.hh
@@ -33,7 +33,8 @@ NS_ASSUME_NONNULL_BEGIN
 + (BOOL)isBufferEmpty;
 + (void)clearBuffer;
 + (void)combineReadingKey:(UniChar)charCode;
-+ (BOOL)checkWhetherToneMarkerConfirms;
++ (BOOL)hasToneMarker;
++ (BOOL)hasToneMarkerOnly;
 + (NSString *)getSyllableComposition;
 + (void)doBackSpaceToBuffer;
 + (NSString *)getComposition;
diff --git a/Source/3rdParty/OVMandarin/Composer.mm b/Source/3rdParty/OVMandarin/Composer.mm
index 09379bc7..6efb5d08 100644
--- a/Source/3rdParty/OVMandarin/Composer.mm
+++ b/Source/3rdParty/OVMandarin/Composer.mm
@@ -52,11 +52,16 @@ static Mandarin::BopomofoReadingBuffer *PhoneticBuffer;
     PhoneticBuffer->combineKey((char)charCode);
 }
 
-+ (BOOL)checkWhetherToneMarkerConfirms
++ (BOOL)hasToneMarker
 {
     return PhoneticBuffer->hasToneMarker();
 }
 
++ (BOOL)hasToneMarkerOnly
+{
+    return PhoneticBuffer->hasToneMarkerOnly();
+}
+
 + (NSString *)getSyllableComposition
 {
     return [NSString stringWithUTF8String:PhoneticBuffer->syllable().composedString().c_str()];
diff --git a/Source/3rdParty/OVMandarin/Mandarin.h b/Source/3rdParty/OVMandarin/Mandarin.h
index 03a46c08..50ae8718 100644
--- a/Source/3rdParty/OVMandarin/Mandarin.h
+++ b/Source/3rdParty/OVMandarin/Mandarin.h
@@ -569,6 +569,12 @@ extern "C" class BopomofoReadingBuffer
         return syllable_.hasToneMarker();
     }
 
+    bool hasToneMarkerOnly() const
+    {
+        return syllable_.hasToneMarker() &&
+               !(syllable_.hasConsonant() || syllable_.hasMiddleVowel() || syllable_.hasVowel());
+    }
+
   protected:
     const BopomofoKeyboardLayout *layout_;
     BPMF syllable_;
diff --git a/Source/Modules/ControllerModules/KeyHandler_HandleInput.swift b/Source/Modules/ControllerModules/KeyHandler_HandleInput.swift
index 8d425a49..cdea1012 100644
--- a/Source/Modules/ControllerModules/KeyHandler_HandleInput.swift
+++ b/Source/Modules/ControllerModules/KeyHandler_HandleInput.swift
@@ -135,23 +135,26 @@ extension KeyHandler {
 
     // MARK: Handle BPMF Keys.
 
-    var composeReading = false
+    var keyConsumedByReading = false
     let skipPhoneticHandling = input.isReservedKey || input.isControlHold || input.isOptionHold
 
     // See if Phonetic reading is valid.
     if !skipPhoneticHandling && Composer.chkKeyValidity(charCode) {
       Composer.combineReadingKey(charCode)
+      keyConsumedByReading = true
 
       // If we have a tone marker, we have to insert the reading to the
       // builder in other words, if we don't have a tone marker, we just
       // update the composing buffer.
-      composeReading = Composer.checkWhetherToneMarkerConfirms()
+      let composeReading = Composer.hasToneMarker()
       if !composeReading {
         stateCallback(buildInputtingState())
         return true
       }
     }
 
+    var composeReading = Composer.hasToneMarker() || Composer.hasToneMarkerOnly()
+
     // See if we have composition if Enter/Space is hit and buffer is not empty.
     // We use "|=" conditioning so that the tone marker key is also taken into account.
     // However, Swift does not support "|=".
@@ -159,10 +162,12 @@ extension KeyHandler {
     if composeReading {
       let reading = Composer.getSyllableComposition()
 
+      // See whether we have a unigram for this...
       if !ifLangModelHasUnigrams(forKey: reading) {
         IME.prtDebugIntel("B49C0979：語彙庫內無「\(reading)」的匹配記錄。")
         errorCallback()
-        stateCallback(buildInputtingState())
+        Composer.clearBuffer()
+        stateCallback((getBuilderLength() == 0) ? InputState.EmptyIgnoringPreviousState() : buildInputtingState())
         return true
       }
 
@@ -210,6 +215,16 @@ extension KeyHandler {
           stateCallback(choosingCandidates)
         }
       }
+      return true  // Telling the client that the key is consumed.
+    }
+
+    // The only possibility for this to be true is that the Bopomofo reading
+    // already has a tone marker but the last key is *not* a tone marker key. An
+    // example is the sequence "6u" with the Standard layout, which produces "ㄧˊ"
+    // but does not compose. Only sequences such as "ㄧˊ", "ˊㄧˊ", "ˊㄧˇ", or "ˊㄧ "
+    // would compose.
+    if keyConsumedByReading {
+      stateCallback(buildInputtingState())
       return true
     }
 
diff --git a/Source/Modules/ControllerModules/KeyHandler_States.swift b/Source/Modules/ControllerModules/KeyHandler_States.swift
index a11cd900..103e02cd 100644
--- a/Source/Modules/ControllerModules/KeyHandler_States.swift
+++ b/Source/Modules/ControllerModules/KeyHandler_States.swift
@@ -303,7 +303,9 @@ extension KeyHandler {
       return false
     }
 
-    if Composer.isBufferEmpty() {
+    if Composer.hasToneMarkerOnly() {
+      Composer.clearBuffer()
+    } else if Composer.isBufferEmpty() {
       if getBuilderCursorIndex() >= 0 {
         deleteBuilderReadingInFrontOfCursor()
         walk()
@@ -462,7 +464,7 @@ extension KeyHandler {
       if !Composer.isBufferEmpty() {
         Composer.clearBuffer()
         if getBuilderLength() == 0 {
-          stateCallback(InputState.Empty())
+          stateCallback(InputState.EmptyIgnoringPreviousState())
         } else {
           stateCallback(buildInputtingState())
         }