diff --git a/overviewer.py b/overviewer.py
index c715a4c..6e755bd 100755
--- a/overviewer.py
+++ b/overviewer.py
@@ -29,17 +29,45 @@ import multiprocessing
 import time
 import logging
 import util
+import platform
 
 logging.basicConfig(level=logging.INFO,format="%(asctime)s [%(levelname)s] %(message)s")
 
+this_dir = util.get_program_path()
+
 # make sure the c_overviewer extension is available
 try:
     import c_overviewer
 except ImportError:
+    ## try to find the build extension
+    ext = os.path.join(this_dir, "c_overviewer.%s" % ("pyd" if platform.system() == "Windows" else "so"))
+    if os.path.exists(ext):
+        print "Something has gone wrong importing the c_overviewer extension.  Please"
+        print "make sure it is up-to-date (clean and rebuild)"
+        sys.exit(1)
+
     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)
 
+if hasattr(sys, "frozen"):
+    pass # we don't bother with a compat test since it should always be in sync
+elif "extension_version" in dir(c_overviewer):
+    # check to make sure the binary matches the headers
+    if os.path.exists(os.path.join(this_dir, "src", "overviewer.h")):
+        with open(os.path.join(this_dir, "src", "overviewer.h")) as f:
+            lines = f.readlines()
+            lines = filter(lambda x: x.startswith("#define OVERVIEWER_EXTENSION_VERSION"), lines)
+            if lines:
+                l = lines[0]
+                if int(l.split()[2].strip()) != c_overviewer.extension_version():
+                    print "Please rebuild your c_overviewer module.  It is out of date!"
+                    sys.exit(1)
+else:
+    print "Please rebuild your c_overviewer module.  It is out of date!"
+    sys.exit(1)
+
+
 import optimizeimages
 import world
 import quadtree
diff --git a/src/main.c b/src/main.c
index e2be7a3..ddbe0af 100644
--- a/src/main.c
+++ b/src/main.c
@@ -17,6 +17,11 @@
 
 #include "overviewer.h"
 
+PyObject *get_extension_version(PyObject *self, PyObject *args) {
+
+    return Py_BuildValue("i", OVERVIEWER_EXTENSION_VERSION);
+}
+
 static PyMethodDef COverviewerMethods[] = {
     {"alpha_over", alpha_over_wrap, METH_VARARGS,
      "alpha over composite function"},
@@ -26,9 +31,12 @@ static PyMethodDef COverviewerMethods[] = {
      "returns available render modes"},
     {"get_render_mode_info", get_render_mode_info, METH_VARARGS,
      "returns info for a particular render mode"},
+    {"extension_version", get_extension_version, METH_VARARGS, 
+        "Returns the extension version"},
     {NULL, NULL, 0, NULL}       /* Sentinel */
 };
 
+
 PyMODINIT_FUNC
 initc_overviewer(void)
 {
diff --git a/src/overviewer.h b/src/overviewer.h
index dc1c31e..c978773 100644
--- a/src/overviewer.h
+++ b/src/overviewer.h
@@ -24,6 +24,10 @@
 #ifndef __OVERVIEWER_H_INCLUDED__
 #define __OVERVIEWER_H_INCLUDED__
 
+// increment this value if you've made a change to the c extesion
+// and want to force users to rebuild
+#define OVERVIEWER_EXTENSION_VERSION 2
+
 /* Python PIL, and numpy headers */
 #include <Python.h>
 #include <Imaging.h>
diff --git a/util.py b/util.py
index f39798a..83f5383 100644
--- a/util.py
+++ b/util.py
@@ -34,11 +34,14 @@ def get_program_path():
 
 
 def findGitVersion():
-    if os.path.exists(".git"):
-        with open(os.path.join(".git","HEAD")) as f:
+    this_dir = get_program_path()
+    if os.path.exists(os.path.join(this_dir,".git")):
+        with open(os.path.join(this_dir,".git","HEAD")) as f:
             data = f.read().strip()
         if data.startswith("ref: "):
-            with open(os.path.join(".git", data[5:])) as g:
+            if not os.path.exists(os.path.join(this_dir,data[5:])):
+                return data
+            with open(os.path.join(this_dir, ".git", data[5:])) as g:
                 return g.read().strip()
         else:
             return data