diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4072b14..5d22310 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -24,7 +24,7 @@ ENDIF (NOT LIBDIR)
 
 SET(REQUIRED_HEADERS
     "ctype.h" "stdbool.h" "stdio.h"
-    "stdint.h" "stdlib.h" "string.h")
+    "stdint.h" "stdlib.h" "string.h" "unistd.h")
 
 FOREACH   (HDR ${REQUIRED_HEADERS})
   CHECK_INCLUDE_FILE (${HDR}  TEST_H)