diff --git a/src/lib/platform/XWindowsScreen.cpp b/src/lib/platform/XWindowsScreen.cpp
index 7f8ce655..1ac5e85e 100644
--- a/src/lib/platform/XWindowsScreen.cpp
+++ b/src/lib/platform/XWindowsScreen.cpp
@@ -1941,15 +1941,15 @@ XWindowsScreen::updateButtons()
 	}
 
 	// allocate button array
-	m_buttons.resize(maxButton);
+	m_buttons.resize(maxButton + 1);
 
 	// fill in button array values.  m_buttons[i] is the physical
-	// button number for logical button i+1.
-	for (UInt32 i = 0; i < numButtons; ++i) {
+	// button number for logical button i.
+	for (UInt32 i = 0; i < maxButton + 1; ++i) {
 		m_buttons[i] = 0;
 	}
 	for (UInt32 i = 0; i < numButtons; ++i) {
-		m_buttons[tmpButtons[i] - 1] = i + 1;
+		m_buttons[tmpButtons[i]] = i;
 	}
 
 	// clean up
diff --git a/src/lib/platform/XWindowsScreen.h b/src/lib/platform/XWindowsScreen.h
index 581e91ff..fd86b3ab 100644
--- a/src/lib/platform/XWindowsScreen.h
+++ b/src/lib/platform/XWindowsScreen.h
@@ -228,7 +228,7 @@ private:
     bool                m_screensaverNotify;
 
     // logical to physical button mapping.  m_buttons[i] gives the
-    // physical button for logical button i+1.
+    // physical button for logical button i.
     std::vector<unsigned char>    m_buttons;
 
     // true if global auto-repeat was enabled before we turned it off