diff --git a/src/if_python3.c b/src/if_python3.c
index 238f968..8958e0a 100644
--- a/src/if_python3.c
+++ b/src/if_python3.c
@@ -306,7 +306,7 @@
  */
 static void end_dynamic_python3(void)
 {
-    if (hinstPy3)
+    if (hinstPy3 != 0)
     {
 	close_dll(hinstPy3);
 	hinstPy3 = 0;
@@ -323,7 +323,17 @@
     int i;
     void *ucs_from_string, *ucs_from_string_and_size;
 
-    if (hinstPy3)
+#if defined(UNIX) && defined(FEAT_PYTHON)
+    /* Can't have Python and Python3 loaded at the same time, it may cause a
+     * crash. */
+    if (python_loaded())
+    {
+	EMSG(_("E999: Python: Cannot use :py and :py3 in one session"));
+	return FAIL;
+    }
+#endif
+
+    if (hinstPy3 != 0)
 	return OK;
     hinstPy3 = load_dll(libname);
 
@@ -506,6 +516,14 @@
     --recurse;
 }
 
+#if (defined(DYNAMIC_PYTHON) && defined(FEAT_PYTHON)) || defined(PROTO)
+    int
+python3_loaded()
+{
+    return (hinstPy3 != 0);
+}
+#endif
+
 static int Python3_Init(void)
 {
     if (!py3initialised)