diff --git a/lib/client/CXWindowsSecondaryScreen.cpp b/lib/client/CXWindowsSecondaryScreen.cpp
index bad3d9f1..ee1cfc0e 100644
--- a/lib/client/CXWindowsSecondaryScreen.cpp
+++ b/lib/client/CXWindowsSecondaryScreen.cpp
@@ -424,7 +424,7 @@ CXWindowsSecondaryScreen::getToggleState() const
 unsigned int
 CXWindowsSecondaryScreen::mapButton(ButtonID id) const
 {
-	if (id < 1 || id > sizeof(m_buttons) / sizeof(m_buttons[0])) {
+	if (id < 1 || id > m_buttons.size()) {
 		// out of range
 		return 0;
 	}
@@ -886,17 +886,34 @@ CXWindowsSecondaryScreen::updateKeys()
 {
 	CDisplayLock display(m_screen);
 
-	// get pointer mapping
-	static const UInt32 maxButtons = sizeof(m_buttons) / sizeof(m_buttons[0]);
-	unsigned char tmpButtons[sizeof(m_buttons) / sizeof(m_buttons[0])];
-	UInt32 numButtons = XGetPointerMapping(display, tmpButtons, maxButtons);
-	for (UInt32 i = 0; i < maxButtons; ++i) {
+	// query the button mapping
+	UInt32 numButtons = XGetPointerMapping(display, NULL, 0);
+	unsigned char* tmpButtons = new unsigned char[numButtons];
+	XGetPointerMapping(display, tmpButtons, numButtons);
+
+	// find the largest logical button id
+	unsigned char maxButton = 0;
+	for (UInt32 i = 0; i < numButtons; ++i) {
+		if (tmpButtons[i] > maxButton) {
+			maxButton = tmpButtons[i];
+		}
+	}
+
+	// allocate button array
+	m_buttons.resize(maxButton);
+
+	// 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) {
 		m_buttons[i] = 0;
 	}
 	for (UInt32 i = 0; i < numButtons; ++i) {
 		m_buttons[tmpButtons[i] - 1] = i + 1;
 	}
 
+	// clean up
+	delete[] tmpButtons;
+
 	// ask server which keys are pressed
 	char keys[32];
 	XQueryKeymap(display, keys);
diff --git a/lib/client/CXWindowsSecondaryScreen.h b/lib/client/CXWindowsSecondaryScreen.h
index 3a9f1284..705f401e 100644
--- a/lib/client/CXWindowsSecondaryScreen.h
+++ b/lib/client/CXWindowsSecondaryScreen.h
@@ -130,7 +130,7 @@ private:
 
 	// logical to physical button mapping.  m_buttons[i] gives the
 	// physical button for logical button i+1.
-	unsigned char		m_buttons[5];
+	std::vector<unsigned char>	m_buttons;
 
 	// current active modifiers (X key masks)
 	unsigned int		m_mask;