diff --git a/superuser/src/main/java/com/topjohnwu/superuser/ShellCallback.java b/superuser/src/main/java/com/topjohnwu/superuser/ShellCallback.java
index b8888b20..d2bbc8b7 100644
--- a/superuser/src/main/java/com/topjohnwu/superuser/ShellCallback.java
+++ b/superuser/src/main/java/com/topjohnwu/superuser/ShellCallback.java
@@ -8,18 +8,19 @@
  * An {@link AbstractList} only used as an abstract container to call {@link #onShellOutput(String)}
  * when a new output is added to the list.
  * <p>
- * The method {@link #onShellOutput(String)} will be called in the thread where the
- * {@link ShellCallbackVector} is constructed by using {@link Handler}. If you need to update
- * the UI, please construct the list in the main thread.
+ * The method {@link #onShellOutput(String)} will be called with a {@link Handler} if the
+ * {@link ShellCallback} is constructed in the main thread (UI thread). If you need to update
+ * the UI, please construct the new instance in the main thread.
  */
 
 public abstract class ShellCallback extends AbstractList<String> implements Shell.IShellCallback {
 
-    private Handler handler;
+    private Handler handler = null;
 
     public ShellCallback() {
         super();
-        handler = new Handler();
+        if (Utils.onMainThread())
+            handler = new Handler();
     }
 
     @Override
@@ -29,12 +30,17 @@ public final int size() {
 
     @Override
     public synchronized boolean add(final String s) {
-        handler.post(new Runnable() {
+        Runnable run = new Runnable() {
             @Override
             public void run() {
                 ShellCallback.this.onShellOutput(s);
             }
-        });
+        };
+        if (handler != null) {
+            handler.post(run);
+        } else {
+            run.run();
+        }
         return true;
     }
 
diff --git a/superuser/src/main/java/com/topjohnwu/superuser/ShellCallbackVector.java b/superuser/src/main/java/com/topjohnwu/superuser/ShellCallbackVector.java
index 1fa61382..855b0f98 100644
--- a/superuser/src/main/java/com/topjohnwu/superuser/ShellCallbackVector.java
+++ b/superuser/src/main/java/com/topjohnwu/superuser/ShellCallbackVector.java
@@ -8,9 +8,9 @@
  * A {@link Vector} to store output of {@link Shell} and call {@link #onShellOutput(String)} when
  * a new output is added to the list.
  * <p>
- * The method {@link #onShellOutput(String)} will be called in the thread where the
- * {@link ShellCallbackVector} is constructed by using {@link Handler}. If you need to update
- * the UI, please construct the list in the main thread.
+ * The method {@link #onShellOutput(String)} will be called with a {@link Handler} if the
+ * {@link ShellCallbackVector} is constructed in the main thread (UI thread). If you need to update
+ * the UI, please construct the new instance in the main thread.
  */
 
 public abstract class ShellCallbackVector extends Vector<String> implements Shell.IShellCallback {
@@ -19,18 +19,24 @@ public abstract class ShellCallbackVector extends Vector<String> implements Shel
 
     public ShellCallbackVector() {
         super();
-        handler = new Handler();
+        if (Utils.onMainThread())
+            handler = new Handler();
     }
 
     @Override
     public boolean add(final String s) {
         boolean ret = super.add(s);
-        handler.post(new Runnable() {
+        Runnable run = new Runnable() {
             @Override
             public void run() {
                 ShellCallbackVector.this.onShellOutput(s);
             }
-        });
+        };
+        if (handler != null) {
+            handler.post(run);
+        } else {
+            run.run();
+        }
         return ret;
     }
 }
diff --git a/superuser/src/main/java/com/topjohnwu/superuser/Utils.java b/superuser/src/main/java/com/topjohnwu/superuser/Utils.java
index acb20f29..9eb80d80 100644
--- a/superuser/src/main/java/com/topjohnwu/superuser/Utils.java
+++ b/superuser/src/main/java/com/topjohnwu/superuser/Utils.java
@@ -1,5 +1,6 @@
 package com.topjohnwu.superuser;
 
+import android.os.Looper;
 import android.util.Log;
 
 import java.security.SecureRandom;
@@ -37,4 +38,8 @@ static void stackTrace(Throwable t) {
         if (hasFlag(Shell.FLAG_VERBOSE_LOGGING))
             t.printStackTrace();
     }
+
+    static boolean onMainThread() {
+        return ((Looper.myLooper() != null) && (Looper.myLooper() == Looper.getMainLooper()));
+    }
 }