--- a/mobile/android/base/FontSizePreference.java
+++ b/mobile/android/base/FontSizePreference.java
@@ -76,27 +76,29 @@ class FontSizePreference extends DialogP
         mScrollingContainer.setBackgroundColor(Color.WHITE);
         mPreviewFontView = (TextView) dialogView.findViewById(R.id.preview);
 
         mDecreaseFontButton = (Button) dialogView.findViewById(R.id.decrease_preview_font_button);
         mIncreaseFontButton = (Button) dialogView.findViewById(R.id.increase_preview_font_button);
         setButtonState(mPreviewFontIndex);
         mDecreaseFontButton.setOnClickListener(new View.OnClickListener() {
             public void onClick(View v) {
-                updatePreviewFontSize(mFontTwipValues[--mPreviewFontIndex]);
+                mPreviewFontIndex = Math.max(mPreviewFontIndex - 1, 0);
+                updatePreviewFontSize(mFontTwipValues[mPreviewFontIndex]);
                 mIncreaseFontButton.setEnabled(true);
                 // If we reached the minimum index, disable the button.
                 if (mPreviewFontIndex == 0) {
                     mDecreaseFontButton.setEnabled(false);
                 }
             }
         });
         mIncreaseFontButton.setOnClickListener(new View.OnClickListener() {
             public void onClick(View v) {
-                updatePreviewFontSize(mFontTwipValues[++mPreviewFontIndex]);
+                mPreviewFontIndex = Math.min(mPreviewFontIndex + 1, mFontTwipValues.length - 1);
+                updatePreviewFontSize(mFontTwipValues[mPreviewFontIndex]);
 
                 mDecreaseFontButton.setEnabled(true);
                 // If we reached the maximum index, disable the button.
                 if (mPreviewFontIndex == mFontTwipValues.length - 1) {
                     mIncreaseFontButton.setEnabled(false);
                 }
             }
         });