diff --git a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3,13 +3,15 @@
 SET(PROJECT_NAME "LEMON")
 PROJECT(${PROJECT_NAME})
 
+INCLUDE(FindPythonInterp)
+
 IF(EXISTS ${PROJECT_SOURCE_DIR}/cmake/version.cmake)
   INCLUDE(${PROJECT_SOURCE_DIR}/cmake/version.cmake)
 ELSEIF(DEFINED ENV{LEMON_VERSION})
   SET(LEMON_VERSION $ENV{LEMON_VERSION} CACHE STRING "LEMON version string.")
 ELSE()
   EXECUTE_PROCESS(
-    COMMAND ./scripts/chg-len.py
+    COMMAND ${PYTHON_EXECUTABLE} ./scripts/chg-len.py
     WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
     OUTPUT_VARIABLE HG_REVISION_PATH
     ERROR_QUIET
@@ -25,7 +27,11 @@
   IF(HG_REVISION STREQUAL "")
     SET(HG_REVISION_ID "hg-tip")
   ELSE()
-    SET(HG_REVISION_ID ${HG_REVISION_PATH}.${HG_REVISION})
+    IF(HG_REVISION_PATH STREQUAL "")
+      SET(HG_REVISION_ID ${HG_REVISION})
+    ELSE()
+      SET(HG_REVISION_ID ${HG_REVISION_PATH}.${HG_REVISION})
+    ENDIF()
   ENDIF()
   SET(LEMON_VERSION ${HG_REVISION_ID} CACHE STRING "LEMON version string.")
 ENDIF()
@@ -44,8 +50,6 @@
 CHECK_TYPE_SIZE("long long" LONG_LONG)
 SET(LEMON_HAVE_LONG_LONG ${HAVE_LONG_LONG})
 
-INCLUDE(FindPythonInterp)
-
 ENABLE_TESTING()
 
 ADD_CUSTOM_TARGET(check COMMAND ${CMAKE_CTEST_COMMAND})