diff --git a/modules/database/src/ioc/rsrv/caservertask.c b/modules/database/src/ioc/rsrv/caservertask.c
index 7a9ae63b3..ea6fb3262 100644
--- a/modules/database/src/ioc/rsrv/caservertask.c
+++ b/modules/database/src/ioc/rsrv/caservertask.c
@@ -1343,8 +1343,15 @@ void casExpandBuffer ( struct message_buffer *buf, ca_uint32_t size, int sendbuf
         // round up to multiple of 4K
         size = ((size-1)|0xfff)+1;
 
-        if (buf->type==mbtLargeTCP)
+        if (buf->type==mbtLargeTCP) {
             newbuf = realloc (buf->buf, size);
+            /*
+             * realloc() copied the data and invalidated the old buffer!
+             * So if we need to copy data from the old buffer, it's now at
+             * the same position in the new buffer.
+             */
+            buf->buf = newbuf;
+        }
         else
             newbuf = malloc (size);
         newtype = mbtLargeTCP;