diff --git a/libraries/AP_Filesystem/AP_Filesystem_Param.cpp b/libraries/AP_Filesystem/AP_Filesystem_Param.cpp
index b5e56e2926..3ab8d687f3 100644
--- a/libraries/AP_Filesystem/AP_Filesystem_Param.cpp
+++ b/libraries/AP_Filesystem/AP_Filesystem_Param.cpp
@@ -55,6 +55,7 @@ int AP_Filesystem_Param::open(const char *fname, int flags)
     r.open = true;
     r.start = 0;
     r.count = 0;
+    r.read_size = 0;
     r.writebuf = nullptr;
     if (!read_only) {
         // setup for upload