diff --git a/HISTORY.md b/HISTORY.md
index 660d6badb..7e5bd4296 100755
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -2,6 +2,9 @@
 
 ## in development:
 
+### 2024-09-13
+* objects, gui: fixed a dictionary lookup glitch for custom block categories 
+
 ## 10.1.2:
 * **Notable Fixes:**
     * only bind rings that replace list items to the list as environment if they are referred to by a non-numerical index (OOP 2.0)
diff --git a/src/gui.js b/src/gui.js
index 0d7949a7a..6ce265a67 100644
--- a/src/gui.js
+++ b/src/gui.js
@@ -1656,7 +1656,7 @@ IDE_Morph.prototype.createCategories = function () {
         var dict = myself.currentSprite.emptyCategories();
         dict.variables = dict.variables || dict.lists || dict.other;
         this.buttons.forEach(cat => {
-            if (dict[cat.category]) {
+            if (Object.hasOwn(dict, cat.category) && (dict[cat.category])) {
                 cat.enable();
             } else {
                 cat.disable();
diff --git a/src/objects.js b/src/objects.js
index 031fa0648..aff9c0a28 100644
--- a/src/objects.js
+++ b/src/objects.js
@@ -184,9 +184,8 @@ SpriteMorph.prototype.allCategories = function () {
 };
 
 SpriteMorph.prototype.blockColorFor = function (category) {
-    return this.blockColor[category] ||
-        this.customCategories.get(category) ||
-        this.blockColor.other;
+    return Object.hasOwn(this.blockColor, category) ? this.blockColor[category]
+        : this.customCategories.get(category) || this.blockColor.other;
 };
 
 SpriteMorph.prototype.paletteColor = new Color(55, 55, 55);
@@ -4158,7 +4157,7 @@ SpriteMorph.prototype.getPrimitiveTemplates = function (category) {
 };
 
 SpriteMorph.prototype.palette = function (category) {
-    if (!this.paletteCache[category]) {
+    if (!Object.hasOwn(this.paletteCache, category)) {
         this.paletteCache[category] = this.freshPalette(category);
     }
     return this.paletteCache[category];