diff --git a/composite.py b/composite.py
index 6cbe44e..be5387f 100644
--- a/composite.py
+++ b/composite.py
@@ -23,12 +23,7 @@ Overviewer. It defaults to the PIL paste function when the custom
 alpha-over extension cannot be found.
 """
 
-extension_alpha_over = None
-try:
-    from c_overviewer import alpha_over as _extension_alpha_over
-    extension_alpha_over = _extension_alpha_over
-except ImportError:
-    pass
+from c_overviewer import alpha_over as extension_alpha_over
 
 def alpha_over(dest, src, pos_or_rect=(0, 0), mask=None):
     """Composite src over dest, using mask as the alpha channel (if
@@ -40,12 +35,4 @@ def alpha_over(dest, src, pos_or_rect=(0, 0), mask=None):
         mask = src
     
     global extension_alpha_over
-    if extension_alpha_over is not None:
-        # extension ALWAYS expects rects, so convert if needed
-        if len(pos_or_rect) == 2:
-            pos_or_rect = (pos_or_rect[0], pos_or_rect[1], src.size[0], src.size[1])
-        extension_alpha_over(dest, src, pos_or_rect, mask)
-    else:
-        # fallback
-        dest.paste(src, pos_or_rect, mask)
-
+    return extension_alpha_over(dest, src, pos_or_rect, mask)
diff --git a/gmap.py b/gmap.py
index 57edb0c..13a24c9 100755
--- a/gmap.py
+++ b/gmap.py
@@ -27,11 +27,19 @@ import re
 import multiprocessing
 import time
 import logging
-import optimizeimages
-import composite
 
 logging.basicConfig(level=logging.INFO,format="%(asctime)s [%(levelname)s] %(message)s")
 
+# make sure the c_overviewer extension is available
+try:
+    import c_overviewer
+except ImportError:
+    print "You need to compile the c_overviewer module to run Minecraft Overviewer."
+    print "Run `python setup.py build`, or see the README for details."
+    sys.exit(1)
+
+import optimizeimages
+import composite
 import world
 import quadtree
 
diff --git a/src/composite.c b/src/composite.c
index 4b58403..1cfdf7c 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -223,6 +223,7 @@ alpha_over_wrap(PyObject *self, PyObject *args)
     /* destination position read */
     if (!PyArg_ParseTuple(pos, "iiii", &dx, &dy, &xsize, &ysize)) {
         /* try again, but this time try to read a point */
+        PyErr_Clear();
         xsize = 0;
         ysize = 0;
         if (!PyArg_ParseTuple(pos, "ii", &dx, &dy)) {