diff --git a/CMakeLists.txt b/CMakeLists.txt
index bca0c22dc0..3abb3dc182 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -304,8 +304,10 @@ include("${CMAKE_CURRENT_SOURCE_DIR}/tools/JoinPaths.cmake")
 # Relative directory setting
 if(USE_PYTHON_INCLUDE_DIR AND DEFINED Python_INCLUDE_DIRS)
   file(RELATIVE_PATH CMAKE_INSTALL_INCLUDEDIR ${CMAKE_INSTALL_PREFIX} ${Python_INCLUDE_DIRS})
+elseif(USE_PYTHON_INCLUDE_DIR AND DEFINED Python3_INCLUDE_DIRS)
+  file(RELATIVE_PATH CMAKE_INSTALL_INCLUDEDIR ${CMAKE_INSTALL_PREFIX} ${Python3_INCLUDE_DIRS})
 elseif(USE_PYTHON_INCLUDE_DIR AND DEFINED PYTHON_INCLUDE_DIR)
-  file(RELATIVE_PATH CMAKE_INSTALL_INCLUDEDIR ${CMAKE_INSTALL_PREFIX} ${PYTHON_INCLUDE_DIRS})
+  file(RELATIVE_PATH CMAKE_INSTALL_INCLUDEDIR ${CMAKE_INSTALL_PREFIX} ${PYTHON_INCLUDE_DIR})
 endif()
 
 if(PYBIND11_INSTALL)
diff --git a/tools/pybind11NewTools.cmake b/tools/pybind11NewTools.cmake
index 5ab3142e47..568b0b0595 100644
--- a/tools/pybind11NewTools.cmake
+++ b/tools/pybind11NewTools.cmake
@@ -112,6 +112,7 @@ if(NOT _PYBIND11_CROSSCOMPILING AND DEFINED ${_Python}_EXECUTABLE)
     # Detect changes to the Python version/binary in subsequent CMake runs, and refresh config if needed
     unset(PYTHON_IS_DEBUG CACHE)
     unset(PYTHON_MODULE_EXTENSION CACHE)
+    unset(PYTHON_MODULE_DEBUG_POSTFIX CACHE)
   endif()
 
   set(PYBIND11_PYTHON_EXECUTABLE_LAST