diff --git a/core/src/processing/core/PImage.java b/core/src/processing/core/PImage.java
index 93a1641e1..2f350775c 100644
--- a/core/src/processing/core/PImage.java
+++ b/core/src/processing/core/PImage.java
@@ -866,6 +866,14 @@ public class PImage implements PConstants, Cloneable {
    */
   public void mask(PImage img) {
     img.loadPixels();
+    if (this.pixelWidth != img.pixelWidth || this.pixelHeight != img.pixelHeight) {
+      if (this.pixelDensity != img.pixelDensity) {
+        throw new IllegalArgumentException("mask() requires the mask image to have the same pixel size after adjusting for pixelDensity.");
+      }
+      else if (this.width != img.width || this.height != img.height) {
+        throw new IllegalArgumentException("mask() requires the mask image to have the same width and height.");
+      }
+    }
     mask(img.pixels);
   }