diff --git a/build/windows/config-cmd.xml b/build/windows/config-cmd.xml
index 67bf1cdf0..44cc9e9ca 100755
--- a/build/windows/config-cmd.xml
+++ b/build/windows/config-cmd.xml
@@ -30,7 +30,7 @@
   <jre>
     <path>java</path>
     <!-- Deal with jokers who install JOGL, ANTLR, etc system-wide -->
-    <opt>-Djava.ext.dirs="%EXEDIR%/java/lib/ext"</opt>
+    <opt>-Djava.ext.dirs="%EXEDIR%\\java\\lib\\ext"</opt>
     <!-- Prevent a user-installed JNA from conflicting with our version.
          https://github.com/processing/processing/issues/2239 -->
     <opt>-Djna.nosys=true</opt>
diff --git a/build/windows/config.xml b/build/windows/config.xml
index 6a376e516..1d7955ca7 100755
--- a/build/windows/config.xml
+++ b/build/windows/config.xml
@@ -27,7 +27,7 @@
   <jre>
     <path>java</path>
     <!-- Deal with jokers who install JOGL, ANTLR, etc system-wide -->
-    <opt>-Djava.ext.dirs="%EXEDIR%/java/lib/ext"</opt>
+    <opt>-Djava.ext.dirs="%EXEDIR%\\java\\lib\\ext"</opt>
     <!-- https://github.com/processing/processing/issues/2239 -->
     <opt>-Djna.nosys=true</opt>
     <!-- Because nosys is set to true, the DLL in the current working 
diff --git a/java/src/processing/mode/java/JavaBuild.java b/java/src/processing/mode/java/JavaBuild.java
index 3cf239c0f..2b939c693 100755
--- a/java/src/processing/mode/java/JavaBuild.java
+++ b/java/src/processing/mode/java/JavaBuild.java
@@ -960,7 +960,7 @@ public class JavaBuild {
                           PApplet.javaVersionName +
                           ".jdk/Contents/Home/jre/lib/ext");
       } else if (exportPlatform == PConstants.WINDOWS) {
-        runOptions.append("-Djava.ext.dirs=\"%EXEDIR%/java/lib/ext\"");
+        runOptions.append("-Djava.ext.dirs=\"%EXEDIR%\\java\\lib\\ext\"");
       } else if (exportPlatform == PConstants.LINUX) {
         runOptions.append("-Djava.ext.dirs=\"$APPDIR/java/lib/ext\"");
       }