diff --git a/c-programming/io/freadln.c b/c-programming/io/freadln.c
index 290b7df..82e20b0 100644
--- a/c-programming/io/freadln.c
+++ b/c-programming/io/freadln.c
@@ -27,8 +27,10 @@ int freadln(FILE* f, char** output, size_t* length_out) {
 	 * 	freadln_ERROR: an error occurred (see errno)
 	 */
 
-	if (output == NULL)
+	if (output == NULL) {
+		errno = EINVAL;
 		return freadln_ERROR;
+	}
 
 	// NOTE: if the file is STDIN, flush STDOUT before waiting for input to
 	// make sure a prompt is displayed