diff --git a/libyara/lexer.c b/libyara/lexer.c
index f232f94768..baf4312e02 100644
--- a/libyara/lexer.c
+++ b/libyara/lexer.c
@@ -3549,6 +3549,9 @@ int yr_lex_parse_rules_bytes(
 
   compiler->errors = 0;
 
+  if (rules_data == NULL)
+    return 0;
+
   if (yylex_init(&yyscanner) != 0)
   {
     compiler->errors = 1;
@@ -3581,6 +3584,9 @@ int yr_lex_parse_rules_string(
 
   compiler->errors = 0;
 
+  if (rules_string == NULL)
+    return 0;
+
   if (yylex_init(&yyscanner) != 0)
   {
     compiler->errors = 1;
diff --git a/libyara/lexer.l b/libyara/lexer.l
index ec6d4c655f..6501436e58 100644
--- a/libyara/lexer.l
+++ b/libyara/lexer.l
@@ -895,6 +895,9 @@ int yr_lex_parse_rules_bytes(
 
   compiler->errors = 0;
 
+  if (rules_data == NULL)
+    return 0;
+
   if (yylex_init(&yyscanner) != 0)
   {
     compiler->errors = 1;
@@ -927,6 +930,9 @@ int yr_lex_parse_rules_string(
 
   compiler->errors = 0;
 
+  if (rules_string == NULL)
+    return 0;
+
   if (yylex_init(&yyscanner) != 0)
   {
     compiler->errors = 1;