diff --git a/src/separator.c b/src/separator.c
index 575011c..bffa2b3 100644
--- a/src/separator.c
+++ b/src/separator.c
@@ -12,6 +12,9 @@ static char *separation_character_line(const char *separation_character, const u
     const size_t character_length = strlen(separation_character);
 
     char *line = malloc(line_length * sizeof(char));
+    // Avoiding undefined behaviour with strlen if no null byte is
+    // present at the end of line
+    *line = 0;
 
     while (strlen(line) + character_length <= line_length) {
         strncat(line, separation_character, character_length);