diff --git a/libraries/AP_Common/ExpandingString.cpp b/libraries/AP_Common/ExpandingString.cpp
index c6a701eec3..bbb7473d0b 100644
--- a/libraries/AP_Common/ExpandingString.cpp
+++ b/libraries/AP_Common/ExpandingString.cpp
@@ -84,16 +84,19 @@ void ExpandingString::printf(const char *format, ...)
 /*
   print into the buffer, expanding if needed
  */
-void ExpandingString::append(const char *s, uint32_t len)
+bool ExpandingString::append(const char *s, uint32_t len)
 {
     if (allocation_failed) {
-        return;
+        return false;
     }
     if (buflen - used < len && !expand(len)) {
-        return;
+        return false;
+    }
+    if (s != nullptr) {
+        memcpy(&buf[used], s, len);
     }
-    memcpy(&buf[used], s, len);
     used += len;
+    return true;
 }
 
 ExpandingString::~ExpandingString()
diff --git a/libraries/AP_Common/ExpandingString.h b/libraries/AP_Common/ExpandingString.h
index 573cb34313..7f4f23629c 100644
--- a/libraries/AP_Common/ExpandingString.h
+++ b/libraries/AP_Common/ExpandingString.h
@@ -36,8 +36,8 @@ public:
     // print into the string
     void printf(const char *format, ...) FMT_PRINTF(2,3);
 
-    // append data to the string
-    void append(const char *s, uint32_t len);
+    // append data to the string. s can be null for zero fill
+    bool append(const char *s, uint32_t len);
 
     // destructor
     ~ExpandingString();