diff --git a/src/libstrongswan/settings/settings_lexer.l b/src/libstrongswan/settings/settings_lexer.l
index 19ab8d7b28..e8c2b9884b 100644
--- a/src/libstrongswan/settings/settings_lexer.l
+++ b/src/libstrongswan/settings/settings_lexer.l
@@ -32,6 +32,11 @@ static void include_files(parser_helper_t *ctx);
 /* do not declare unneeded functions */
 %option noinput noyywrap
 
+/* do not include unistd.h as it might conflict with our scanner states */
+%option nounistd
+/* due to that disable interactive mode, which requires isatty() */
+%option never-interactive
+
 /* don't use global variables, and interact properly with bison */
 %option reentrant bison-bridge
 
diff --git a/src/starter/parser/lexer.l b/src/starter/parser/lexer.l
index fb23a0f931..b81d6ce74d 100644
--- a/src/starter/parser/lexer.l
+++ b/src/starter/parser/lexer.l
@@ -33,6 +33,11 @@ static void include_files(parser_helper_t *ctx);
 /* do not declare unneeded functions */
 %option noinput noyywrap
 
+/* do not include unistd.h as it might conflict with our scanner states */
+%option nounistd
+/* due to that disable interactive mode, which requires isatty() */
+%option never-interactive
+
 /* don't use global variables, and interact properly with bison */
 %option reentrant bison-bridge