diff --git a/Tools/AP_Bootloader/support.cpp b/Tools/AP_Bootloader/support.cpp
index 993c89ceb3..82971ead2f 100644
--- a/Tools/AP_Bootloader/support.cpp
+++ b/Tools/AP_Bootloader/support.cpp
@@ -283,6 +283,9 @@ int strncmp(const char *s1, const char *s2, size_t n)
         s1++;
         s2++;
     }
+    if (n == 0) {
+        return 0;
+    }
     return (*s1 - *s2);
 }