diff --git a/src/msw/window.cpp b/src/msw/window.cpp
index 8a34b50fa9..246f47ba11 100644
--- a/src/msw/window.cpp
+++ b/src/msw/window.cpp
@@ -1787,6 +1787,15 @@ void wxWindowMSW::DoGetClientSize(int *x, int *y) const
         if ( y )
             *y = rect.bottom;
     }
+
+    // The size of the client window can't be negative but ::GetClientRect()
+    // can return negative size for an extremely small (1x1) window with
+    // borders so ensure that we correct it here as having negative sizes is
+    // completely unexpected.
+    if ( x && *x < 0 )
+        *x = 0;
+    if ( y && *y < 0 )
+        *y = 0;
 }
 
 void wxWindowMSW::DoGetPosition(int *x, int *y) const
diff --git a/tests/window/clientsize.cpp b/tests/window/clientsize.cpp
index 1b075fa1f6..57089680ef 100644
--- a/tests/window/clientsize.cpp
+++ b/tests/window/clientsize.cpp
@@ -37,10 +37,12 @@ public:
 private:
     CPPUNIT_TEST_SUITE( ClientSizeTestCase );
         CPPUNIT_TEST( ClientToWindow );
+        CPPUNIT_TEST( ClientSizeNotNegative );
         CPPUNIT_TEST( WindowToClient );
     CPPUNIT_TEST_SUITE_END();
 
     void ClientToWindow();
+    void ClientSizeNotNegative();
     void WindowToClient();
 
     wxWindow *m_win;
@@ -78,6 +80,18 @@ void ClientSizeTestCase::ClientToWindow()
                    m_win->ClientToWindowSize(m_win->GetClientSize()));
 }
 
+void ClientSizeTestCase::ClientSizeNotNegative()
+{
+    wxWindow* w = new wxWindow(wxTheApp->GetTopWindow(), -1,
+                               wxDefaultPosition, wxDefaultSize,
+                               wxBORDER_THEME);
+    w->SetSize(wxSize(1,1));
+    const wxSize szw = w->GetClientSize();
+    CPPUNIT_ASSERT(szw.GetWidth() >= 0);
+    CPPUNIT_ASSERT(szw.GetHeight() >= 0);
+    w->Destroy();
+}
+
 void ClientSizeTestCase::WindowToClient()
 {
     CPPUNIT_ASSERT(m_win->GetClientSize() ==