diff --git a/sapi/aolserver/aolserver.c b/sapi/aolserver/aolserver.c
index cbc4ec02573..c76ea19d45a 100644
--- a/sapi/aolserver/aolserver.c
+++ b/sapi/aolserver/aolserver.c
@@ -18,13 +18,17 @@
 
 /* $Id$ */
 
-/* conflict between PHP and aolserver */
+/* conflict between PHP and AOLserver */
 #define Debug php_Debug
 #include "php.h"
 #undef Debug
 
 #ifdef HAVE_AOLSERVER
 
+#ifndef ZTS
+#error AOLserver module only useable in thread-safe mode
+#endif
+
 #include "php_ini.h"
 #include "php_globals.h"
 #include "SAPI.h"
diff --git a/sapi/nsapi/aolserver.c b/sapi/nsapi/aolserver.c
index cbc4ec02573..c76ea19d45a 100644
--- a/sapi/nsapi/aolserver.c
+++ b/sapi/nsapi/aolserver.c
@@ -18,13 +18,17 @@
 
 /* $Id$ */
 
-/* conflict between PHP and aolserver */
+/* conflict between PHP and AOLserver */
 #define Debug php_Debug
 #include "php.h"
 #undef Debug
 
 #ifdef HAVE_AOLSERVER
 
+#ifndef ZTS
+#error AOLserver module only useable in thread-safe mode
+#endif
+
 #include "php_ini.h"
 #include "php_globals.h"
 #include "SAPI.h"