diff --git a/lib/file.c b/lib/file.c
index 4b1017c6f..dc458db1f 100644
--- a/lib/file.c
+++ b/lib/file.c
@@ -360,7 +360,7 @@ static CURLcode file_upload(struct connectdata *conn)
 
   while(!result) {
     int readcount;
-    result = Curl_fillreadbuffer(conn, BUFSIZE, &readcount);
+    result = Curl_fillreadbuffer(conn, (int)data->set.buffer_size, &readcount);
     if(result)
       break;
 
diff --git a/lib/smb.c b/lib/smb.c
index 51b3434cb..5b1ffa9b7 100644
--- a/lib/smb.c
+++ b/lib/smb.c
@@ -607,7 +607,7 @@ static CURLcode smb_send_and_recv(struct connectdata *conn, void **msg)
 
   /* Check if there is data in the transfer buffer */
   if(!smbc->send_size && smbc->upload_size) {
-    int nread = smbc->upload_size > BUFSIZE ? BUFSIZE :
+    int nread = smbc->upload_size > UPLOAD_BUFSIZE ? UPLOAD_BUFSIZE :
       (int) smbc->upload_size;
     conn->data->req.upload_fromhere = conn->data->state.uploadbuffer;
     result = Curl_fillreadbuffer(conn, nread, &nread);
diff --git a/lib/transfer.c b/lib/transfer.c
index d6aac675b..bafb6e9c8 100644
--- a/lib/transfer.c
+++ b/lib/transfer.c
@@ -904,7 +904,7 @@ static CURLcode readwrite_upload(struct Curl_easy *data,
             sending_http_headers = FALSE;
         }
 
-        result = Curl_fillreadbuffer(conn, BUFSIZE, &fillcount);
+        result = Curl_fillreadbuffer(conn, UPLOAD_BUFSIZE, &fillcount);
         if(result)
           return result;
 
diff --git a/lib/urldata.h b/lib/urldata.h
index 6e87684bf..22b0f8195 100644
--- a/lib/urldata.h
+++ b/lib/urldata.h
@@ -207,6 +207,10 @@
 #define MAX_BUFSIZE CURL_MAX_READ_SIZE
 #define MIN_BUFSIZE 1024
 
+/* The upload buffer size, should not be smaller than CURL_MAX_WRITE_SIZE, as
+   it needs to hold a full buffer as could be sent in a write callback */
+#define UPLOAD_BUFSIZE CURL_MAX_WRITE_SIZE
+
 /* Initial size of the buffer to store headers in, it'll be enlarged in case
    of need. */
 #define HEADERSIZE 256
@@ -1333,7 +1337,7 @@ struct UrlState {
   size_t headersize;   /* size of the allocation */
 
   char *buffer; /* download buffer */
-  char uploadbuffer[BUFSIZE+1]; /* upload buffer */
+  char uploadbuffer[UPLOAD_BUFSIZE+1]; /* upload buffer */
   curl_off_t current_speed;  /* the ProgressShow() function sets this,
                                 bytes / second */
   bool this_is_a_follow; /* this is a followed Location: request */