When assembler source contains "nop" (no-operation) instruction without an operand, gas on s390x produces an error

a.s: Assembler messages:
a.s:2: Error: missing operand

In my opinion "nop" should translate into a no-operation machine instruction without requiring any operand in the assembler sources.

Version-Release number of selected component (if applicable):
binutils-2.19.51.0.2-17.fc11

also present in RHEL-4 and RHEL-5
binutils-2.15.92.0.2-24 (EL4)
binutils-2.17.50.0.6-12.el5


Reproducer is:
as << EOF
  .section code
  nop
EOF

and it results into:
{standard input}: Assembler messages:
{standard input}:2: Error: missing operand


Aditional info:

Real world example of breakage is build of postgresql (https://s390.koji.fedoraproject.org/koji/taskinfo?taskID=82106) with static probes where the probe (from sys/sdt.h) translates into

======
.LVL471:
#APP
# 1551 "xact.c" 1
        .section .probes
        .align 8
1:
        .asciz "transaction__start"
        .align 4
        .int 0x31425250
        .align 8
        .long 1b
        .align 8
        .long 2f
        .previous

# 0 "" 2
#NO_APP
        l       %r1,164(%r15)
#APP
# 1551 "xact.c" 1
        2:
        nop /* %r1 */
# 0 "" 2
#NO_APP
=====
and

xact.c: Assembler messages:
xact.c:2089: Error: missing operand
xact.c:1552: Error: missing operand
xact.c:1679: Error: missing operand


These lines from opcodes/s390-opc.txt are related to "nop":
0700 nopr RR_0R "no operation" g5 esa,zarch
0700 b*8r RR_0R "conditional branch" g5 esa,zarch
4700 nop RX_0RRD "no operation" g5 esa,zarch
4700 b*8 RX_0RRD "conditional branch" g5 esa,zarch

Even the Linux kernel defines a nop in /arch/s390/include/asm/system.h as
#define nop() asm volatile("nop")
but it's not probably used, otherwise it would break the build.