diff --git a/HelperFunctions.c b/HelperFunctions.c
index 3695395c6b759d7d676a1c44a151ffaac8e9d417..53790efb60791ff081ff99a30d417a0146d82c9d 100644
--- a/HelperFunctions.c
+++ b/HelperFunctions.c
@@ -40,6 +40,10 @@
  * Return: Copy of source string.
  */
 char* copy_string(char* source_ptr) {
+    // Check for valid input.
+    if (source_ptr == NULL) {
+        return NULL;
+    }
     char* copy_ptr;
     copy_ptr = calloc((strlen(source_ptr) + 1), sizeof(char));
     if (copy_ptr != NULL) {
@@ -58,6 +62,10 @@ char* copy_string(char* source_ptr) {
  * Return: Copy of source string.
  */
 char* copy_string_with_buffer(char* source_ptr, int buffer_size) {
+    // Check for valid input.
+    if (source_ptr == NULL) {
+        return NULL;
+    }
     char* copy_ptr;
     copy_ptr = calloc(buffer_size, sizeof(char));
     if (copy_ptr != NULL) {
@@ -130,6 +138,10 @@ double* copy_double(double* source_ptr) {
  * Return: Lowercase version of initial string.
  */
 char* to_lower_case(char* input_string) {
+    // Check for valid input.
+    if (input_string == NULL) {
+        return NULL;
+    }
     int index = 0;
     char* return_string = calloc((strlen(input_string) + 1), sizeof(char));
     while (input_string[index]) {
@@ -146,6 +158,10 @@ char* to_lower_case(char* input_string) {
  * Return: Uppercase version of initial string.
  */
 char* to_upper_case(char* input_string) {
+    // Check for valid input.
+    if (input_string == NULL) {
+        return NULL;
+    }
     int index = 0;
     char* return_string = calloc((strlen(input_string) + 1), sizeof(char));
     while (input_string[index]) {
@@ -164,6 +180,10 @@ char* to_upper_case(char* input_string) {
  * Return: Converted version of initial string.
  */
 char* first_letter_upper(char* input_string) {
+    // Check for valid input.
+    if (input_string == NULL) {
+        return NULL;
+    }
     int index = 0;
     char* return_string = calloc((strlen(input_string) + 1), sizeof(char));
     while (input_string[index]) {
@@ -188,6 +208,10 @@ char* first_letter_upper(char* input_string) {
  * Return: Quote-less version of initial string.
  */
 char* remove_quotes(char* input_string) {
+    // Check for valid input.
+    if (input_string == NULL) {
+        return NULL;
+    }
     size_t string_length = strlen(input_string);
     int orig_index;
     int replace_index = 0;
@@ -215,6 +239,10 @@ char* remove_quotes(char* input_string) {
  * Return: Newline-less version of initial string.
  */
 char* remove_newline(char* input_string) {
+    // Check for valid input.
+    if (input_string == NULL) {
+        return NULL;
+    }
     size_t string_length = strlen(input_string);
     int orig_index;
     int replace_index = 0;