diff --git a/ncc/include/stdlib.h b/ncc/include/stdlib.h
index 08be9a5..e1caf9e 100644
--- a/ncc/include/stdlib.h
+++ b/ncc/include/stdlib.h
@@ -21,13 +21,22 @@ char* itoa(int value, char* str, int base)
 {
     assert(base > 0 && base <= 16);
 
+    // If negative, write a minus sign
+    if (value < 0)
+    {
+        *str = '-';
+        ++str;
+        value = -value;
+    }
+
     // Compute the number of digits
     int num_digits = 0;
-    for (int n = value; n > 0; n = n / base)
+    int n = value;
+    do
     {
-        int digit = value % base;
+        n = n / base;
         ++num_digits;
-    }
+    } while (n > 0);
 
     // The digits have to be written in reverse order
     for (int i = num_digits - 1; i >= 0; --i)
diff --git a/ncc/tests/stdlib.c b/ncc/tests/stdlib.c
index 321b565..6131eac 100644
--- a/ncc/tests/stdlib.c
+++ b/ncc/tests/stdlib.c
@@ -32,6 +32,13 @@ void main()
     assert(buf[0] == '1' && buf[1] == '7' && buf[2] == 0);
     itoa(15, buf, 16);
     assert(buf[0] == 'F' && buf[1] == 0);
+    itoa(-77, buf, 10);
+    assert(buf[0] == '-');
+    assert(buf[1] == '7');
+    assert(buf[2] == '7');
+    itoa(0, buf, 10);
+    assert(buf[0] == '0');
+    assert(buf[1] == '\0');
 
     // Test the exit function
     exit(0);