diff --git a/backend/sharp.c b/backend/sharp.c
index da7af8fa6..6479b3d60 100644
--- a/backend/sharp.c
+++ b/backend/sharp.c
@@ -107,8 +107,10 @@
 */
 
 #ifdef HAVE_SYS_SHM_H
+#ifndef HAVE_OS2_H
 #define USE_FORK
 #endif
+#endif
 
 #ifdef USE_FORK
 #include <signal.h>