diff --git a/ext/session/config.m4 b/ext/session/config.m4
index 949306c0268..b33855a6769 100644
--- a/ext/session/config.m4
+++ b/ext/session/config.m4
@@ -27,7 +27,12 @@ if test "$PHP_MM" != "no"; then
   if test -z "$MM_DIR" ; then
     AC_MSG_ERROR(cannot find mm library)
   fi
-  
+
+  if test "$enable_maintainer_zts" = "yes"; then
+    dnl The mm library is not thread-safe, and mod_mm.c refuses to compile.
+    AC_MSG_ERROR(--with-mm cannot be combined with --enable-maintainer-zts)
+  fi
+
   PHP_ADD_LIBRARY_WITH_PATH(mm, $MM_DIR/$PHP_LIBDIR, SESSION_SHARED_LIBADD)
   PHP_ADD_INCLUDE($MM_DIR/include)
   PHP_INSTALL_HEADERS([ext/session/mod_mm.h])