diff --git a/emcclib.js b/emcclib.js
index 1be8979..3824293 100644
--- a/emcclib.js
+++ b/emcclib.js
@@ -224,7 +224,6 @@ mergeInto(LibraryManager.library, {
      * Prepare to do some drawing on the canvas.
      */
     js_canvas_start_draw: function() {
-        ctx = offscreen_canvas.getContext('2d', { alpha: false });
         update_xmin = update_xmax = update_ymin = update_ymax = undefined;
     },
 
@@ -267,7 +266,6 @@ mergeInto(LibraryManager.library, {
                                    update_xmax - update_xmin,
                                    update_ymax - update_ymin);
         }
-        ctx = null;
     },
 
     /*
diff --git a/emccpre.js b/emccpre.js
index bba40bc..6fba9b2 100644
--- a/emccpre.js
+++ b/emccpre.js
@@ -22,7 +22,7 @@
 var onscreen_canvas, offscreen_canvas;
 
 // A persistent drawing context for the offscreen canvas, to save
-// constructing one per individual graphics operation.
+// requesting it for each individual graphics operation.
 var ctx;
 
 // Bounding rectangle for the copy to the onscreen canvas that will be
@@ -245,6 +245,7 @@ function initPuzzle() {
     // Construct the off-screen canvas used for double buffering.
     onscreen_canvas = document.getElementById("puzzlecanvas");
     offscreen_canvas = document.createElement("canvas");
+    ctx = offscreen_canvas.getContext('2d', { alpha: false });
 
     // Stop right-clicks on the puzzle from popping up a context menu.
     // We need those right-clicks!