diff --git a/main.c b/main.c
index da2813f..8112711 100644
--- a/main.c
+++ b/main.c
@@ -28,14 +28,17 @@
  
 /*!
  * String length calculation.
- * @arg     R0: Pointer to string
- * @return  R0: String length (unsigned)
+ * @arg     src{R0}     Pointer to string
+ * @return  {R0}        String length, -1 if src is NULL
  */
 __asm uint32_t strLength (const char* src) {
     // R0: length, return value
     // R1: src
     // R2: v = *src
     MOV     R1, R0          // Get variable
+    CMP     R1, #0          // if (src{R1} == NULL)
+    MOVEQ   R0, #-1         //    return -1;
+    BXEQ    LR
     MOV     R0, #0          // length{R0} =0;
 len_loop                    // do {
     LDRB    R2, [R1], #1    //    v{R2} = *src++;
@@ -62,6 +65,8 @@ __asm uint32_t isPalindrome (const char* src) {
     // R3: v1 = src[i]
     // R4: v2 = src[j]
     PUSH    {R4, LR}
+    CMP     R0, #0          // if (src{R0} == NULL)
+    BEQ     pal_false       //    return false;
     MOV     R4, R0          // save R0
     BL      strLength       // j{R2} = strLength(src) - 1
     SUB     R2, R0, #1
@@ -77,7 +82,7 @@ pal_loop
     ADDEQ   R1, R1, #1      //       ++i;
     SUBEQ   R2, R2, #1      //       --j;
     BEQ     pal_loop        //       // goto loop;
-                            //    } else
+pal_false                   //    } else
     MOV     R0, #0          //       return *RESULT = 0;
     LDR     R1, =RESULT      
     STRB    R0, [R1]
@@ -91,14 +96,20 @@ pal_true
 }
 
 /*!
- * Main routine
+ * Main routine.
+ * @note
+ *      This routine plays the role of debuging-mode unit testing.
  */
 int main(void) {
     const char *s1 = "hello world!";    // Not a palindrome
-    const char *s2 = "aibohphobia";     // Aibophobia def= "Fear of palindromes", palindrome.
+    const char *s2 = "aibohphobia";     // Aibophobia def= "Irational fear of palindromes", palindrome.
+    const char *s3 = "";                // Empty string, palindrome.
+    const char *s4 = 0;                 // NULL pointer. Out of scope, not palindrome
 
     uint32_t res1 = isPalindrome (s1);  // Test case 1
     uint32_t res2 = isPalindrome (s2);  // Test case 2
+    uint32_t res3 = isPalindrome (s3);  // Test case 3
+    uint32_t res4 = isPalindrome (s4);  // Test case 4
     
     while (1);          // Trap forever
     //return 0;         //!< comment out return, to suppress "statement is unreachable" warning.