diff --git a/gui.js b/gui.js
index 340f2429..6c9cdc5f 100644
--- a/gui.js
+++ b/gui.js
@@ -75,7 +75,7 @@ isRetinaSupported, SliderMorph, Animation*/
 
 // Global stuff ////////////////////////////////////////////////////////
 
-modules.gui = '2017-November-16';
+modules.gui = '2017-November-26';
 
 // Declarations
 
@@ -7203,22 +7203,26 @@ SpriteIconMorph.prototype.reactToDropOf = function (morph, hand) {
 };
 
 SpriteIconMorph.prototype.copyStack = function (block) {
-    var dup = block.fullCopy(),
-        y = Math.max(this.object.scripts.children.map(function (stack) {
+    var sprite = this.object,
+    	dup = block.fullCopy(),
+        y = Math.max(sprite.scripts.children.map(function (stack) {
             return stack.fullBounds().bottom();
-        }).concat([this.object.scripts.top()]));
+        }).concat([sprite.scripts.top()]));
 
-    dup.setPosition(new Point(this.object.scripts.left() + 20, y + 20));
-    this.object.scripts.add(dup);
+    dup.setPosition(new Point(sprite.scripts.left() + 20, y + 20));
+    sprite.scripts.add(dup);
     dup.allComments().forEach(function (comment) {
         comment.align(dup);
     });
-    this.object.scripts.adjustBounds();
+    sprite.scripts.adjustBounds();
 
-    // delete all custom blocks pointing to local definitions
-    // under construction...
+    // delete all local custom blocks (methods) that the receiver
+    // doesn't understand
     dup.allChildren().forEach(function (morph) {
-        if (morph.definition && !morph.definition.isGlobal) {
+	    if (morph.isCustomBlock &&
+        		!morph.isGlobal &&
+        		!sprite.getMethod(morph.blockSpec)
+        ) {
             morph.deleteBlock();
         }
     });
diff --git a/history.txt b/history.txt
index d97713e4..72d4e80c 100755
--- a/history.txt
+++ b/history.txt
@@ -3802,3 +3802,7 @@ Fixes:
 * Blocks: avoid coloring the block-highlight when re-coloring a syntax element, this prevents highlighted blocks inside LAUNCH statements to expand when repeatedly single-stepped.
 
 === v4.1.0.4 maintenance release ===
+
+171126
+------
+* GUI: fixed #1933 - avoid creating "obsolete" blocks by not copying method blocks into sprites that don't understand them