diff --git a/app/src/processing/app/Base.java b/app/src/processing/app/Base.java
index 898397b88..59be722ce 100644
--- a/app/src/processing/app/Base.java
+++ b/app/src/processing/app/Base.java
@@ -1644,6 +1644,12 @@ public class Base {
     return PConstants.platformNames[PApplet.platform];
   }
 
+
+  /**
+   * Return whether sketches will run as 32- or 64-bits. On Linux and Windows, 
+   * this is the bit depth of the machine, while on OS X it's determined by the
+   * setting from preferences, since both 32- and 64-bit are supported.
+   */
   static public int getNativeBits() {
     if (Base.isMacOS()) {
       return Preferences.getInteger("run.options.bits");
diff --git a/app/src/processing/mode/java/runner/Runner.java b/app/src/processing/mode/java/runner/Runner.java
index 3453f3cad..219787e21 100644
--- a/app/src/processing/mode/java/runner/Runner.java
+++ b/app/src/processing/mode/java/runner/Runner.java
@@ -84,7 +84,7 @@ public class Runner implements MessageConsumer {
 //  private String appletClassName;
 
 
-  public Runner(JavaBuild build, RunnerListener listener) {
+  public Runner(JavaBuild build, RunnerListener listener) throws SketchException {
     this.listener = listener;
 //    this.sketch = sketch;
     this.build = build;
@@ -94,6 +94,14 @@ public class Runner implements MessageConsumer {
 //    } else {
 //      System.out.println("actually it's a " + listener.getClass().getName());
     }
+
+    // Make sure all the imported libraries will actually run with this setup.
+    int bits = Base.getNativeBits();
+    for (Library library : build.getImportedLibraries()) {
+      if (!library.supportsArch(PApplet.platform, bits)) {
+        throw new SketchException(library.getName() + " does not run in " + bits + "-bit mode.");
+      }
+    }
   }
 
 
@@ -281,7 +289,7 @@ public class Runner implements MessageConsumer {
         "java -Xrunjdwp:transport=dt_shmem,address=" + addr + ",suspend=y ";
     } else if (Base.isMacOS()) {
       commandArgs =
-        "java -d" + Preferences.get("run.options.bits") +  
+        "java -d" + Base.getNativeBits() + //Preferences.get("run.options.bits") +  
         " -Xrunjdwp:transport=dt_socket,address=" + addr + ",suspend=y ";
     }