diff --git a/tests/server/sockfilt.c b/tests/server/sockfilt.c
index 333e02f26..4b9fec4ef 100644
--- a/tests/server/sockfilt.c
+++ b/tests/server/sockfilt.c
@@ -290,6 +290,7 @@ static int juggle(curl_socket_t *sockfdp,
 
         if(5 != read(fileno(stdin), buffer, 5))
           return FALSE;
+        buffer[4] = '\0';
 
         len = (ssize_t)strtol((char *)buffer, NULL, 16);
         if(len != read(fileno(stdin), buffer, len))