diff --git a/gui.js b/gui.js
index de5bc37b..c9e05f16 100644
--- a/gui.js
+++ b/gui.js
@@ -67,7 +67,7 @@ AlignmentMorph, TabMorph, Costume, MorphicPreferences, Sound, BlockMorph,
 ToggleMorph, InputSlotDialogMorph, ScriptsMorph, isNil, SymbolMorph,
 BlockExportDialogMorph, BlockImportDialogMorph, SnapTranslator, localize,
 List, ArgMorph, SnapCloud, Uint8Array, HandleMorph, SVG_Costume,
-fontHeight, hex_sha512, sb, CommentMorph, CommandBlockMorph,
+fontHeight, hex_sha512, sb, CommentMorph, CommandBlockMorph, BooleanSlotMorph,
 BlockLabelPlaceHolderMorph, Audio, SpeechBubbleMorph, ScriptFocusMorph,
 XML_Element, WatcherMorph, BlockRemovalDialogMorph, saveAs, TableMorph,
 isSnapObject, isRetinaEnabled, disableRetinaSupport, enableRetinaSupport,
@@ -75,7 +75,7 @@ isRetinaSupported, SliderMorph, Animation*/
 
 // Global stuff ////////////////////////////////////////////////////////
 
-modules.gui = '2017-September-26';
+modules.gui = '2017-September-28';
 
 // Declarations
 
@@ -2651,6 +2651,17 @@ IDE_Morph.prototype.settingsMenu = function () {
         'check to turn on\n visible stepping (slow)',
         false
     );
+    addPreference(
+        'Ternary Boolean slots',
+        function () {
+        	BooleanSlotMorph.prototype.isTernary =
+        		!BooleanSlotMorph.prototype.isTernary;
+      	},
+        BooleanSlotMorph.prototype.isTernary,
+        'uncheck to limit\nBoolean slots to true / false',
+        'check to allow\nempty Boolean slots',
+        true
+    );
     addPreference(
         'Camera support',
         'toggleCameraSupport',
diff --git a/history.txt b/history.txt
index 2b40aa6b..eb22ee71 100755
--- a/history.txt
+++ b/history.txt
@@ -3664,6 +3664,10 @@ Fixes:
 
 === v4.1 release candidate ===
 
+170928
+------
+* GUI: enable experimental setting for "ternary Boolean slots"
+
 
 v4.1 Features:
 * polymorphic sprite-local custom blocks