diff --git a/src/file_parser.rs b/src/file_parser.rs
index 2a497a2..8dd8afc 100644
--- a/src/file_parser.rs
+++ b/src/file_parser.rs
@@ -10,7 +10,7 @@ use crate::line_parser::LineType;
 pub struct Filelist {
     pub files: Vec<String>,
     pub incdirs: Vec<String>,
-    pub defines: HashMap<String, String>,
+    pub defines: HashMap<String, Option<String>>,
     pub comments_present: bool,
 }
 
@@ -42,8 +42,11 @@ pub fn parse_file(path: &str) -> Result<Filelist, Box<dyn Error>> {
         match line_parser::parse_line(&line) {
             LineType::File(file) => filelist.files.push(file.to_string()),
             LineType::Define(define_map) => {
-                for define in define_map.into_iter() {
-                    filelist.defines.insert(define.0.to_string(), define.1.to_string());
+                for (d, t) in define_map.into_iter() {
+                    match t {
+                        Some(text) => filelist.defines.insert(d.to_string(), Some(text.to_string())),
+                        None => filelist.defines.insert(d.to_string(), None)
+                    };
                 }
             },
             LineType::IncDir(incdirs) => {
diff --git a/src/line_parser.rs b/src/line_parser.rs
index f128639..a7f63fa 100644
--- a/src/line_parser.rs
+++ b/src/line_parser.rs
@@ -4,7 +4,7 @@ use std::collections::HashMap;
 pub enum LineType<'a> {
     File(&'a str),
     IncDir(Vec<&'a str>),
-    Define(HashMap<&'a str, &'a str>),
+    Define(HashMap<&'a str, Option<&'a str>>),
     Filelist(&'a str),
     Comment,
     // Unknown,
@@ -20,8 +20,12 @@ pub fn parse_line(line: &str) -> LineType {
         let defines = line.trim_start_matches("+define+").trim_end_matches('+');
         let mut define_map = HashMap::new();
         for define in defines.split('+') {
-            let split: Vec<&str> = define.splitn(2, '=').collect();
-            define_map.insert(split[0], split[1]);
+            if let Some(pos) = define.find("=") {
+                let (d, t) = define.split_at(pos);
+                define_map.insert(d, Some(&t[1..]));
+            } else {
+                define_map.insert(define, None);
+            }
         }
         LineType::Define(define_map)
     } else if line.starts_with("+incdir+") {
@@ -49,19 +53,21 @@ mod test {
 
     #[test]
     fn parse_line_define_single() {
-        let line = "+define+CONST1=const1=23\n";
+        let line = "+define+CONST1=const1=23+\n";
         let mut define_map = HashMap::new();
-        define_map.insert("CONST1", "const1=23");
+        define_map.insert("CONST1", Some("const1=23"));
         assert_eq!(parse_line(line), LineType::Define(define_map));
     }
 
     #[test]
     fn parse_line_define_multiple() {
-        let line = "+define+CONST1=const1+CONST2=const2+CONST3=const3=1+\n";
+        let line = "+define+CONST1=const1+CONST2=const2+CONST3=const3=1+CONST4+CONST5+\n";
         let mut define_map = HashMap::new();
-        define_map.insert("CONST1", "const1");
-        define_map.insert("CONST2", "const2");
-        define_map.insert("CONST3", "const3=1");
+        define_map.insert("CONST1", Some("const1"));
+        define_map.insert("CONST2", Some("const2"));
+        define_map.insert("CONST3", Some("const3=1"));
+        define_map.insert("CONST4", None);
+        define_map.insert("CONST5", None);
         assert_eq!(parse_line(line), LineType::Define(define_map));
     }
 
diff --git a/testcase/files.f b/testcase/files.f
index 4eecee6..52cce6e 100644
--- a/testcase/files.f
+++ b/testcase/files.f
@@ -6,4 +6,5 @@ testcase/file3.sv
 +define+a=b+c=d+e=f
 +define+$(VAR1)=var1
 +define+${VAR2}=var2
++define+RTL
 -f testcase/files2.f
\ No newline at end of file
diff --git a/tests/integration_tests.rs b/tests/integration_tests.rs
index 93a0723..6e95fcd 100644
--- a/tests/integration_tests.rs
+++ b/tests/integration_tests.rs
@@ -4,12 +4,12 @@ use std::collections::HashMap;
 #[test]
 fn simple_test() {
     let mut defines = HashMap::new();
-    defines.insert("a".to_string(), "bad".to_string());
-    defines.insert("e".to_string(), "f".to_string());
-    defines.insert("c".to_string(), "d".to_string());
-    defines.insert("ENV_VAR1".to_string(), "var1".to_string());
-    defines.insert("ENV_VAR2".to_string(), "var2".to_string());
-
+    defines.insert("a".to_string(), Some("bad".to_string()));
+    defines.insert("e".to_string(), Some("f".to_string()));
+    defines.insert("c".to_string(), Some("d".to_string()));
+    defines.insert("ENV_VAR1".to_string(), Some("var1".to_string()));
+    defines.insert("ENV_VAR2".to_string(), Some("var2".to_string()));
+    defines.insert("RTL".to_string(), None);
 
     let filelist_exp = sv_filelist_parser::Filelist {
         files : vec!["testcase/file1.sv".to_string(),