diff --git a/gas/config/tc-aarch64.c b/gas/config/tc-aarch64.c
index 67c0871..6c476d7 100644
--- a/gas/config/tc-aarch64.c
+++ b/gas/config/tc-aarch64.c
@@ -42,6 +42,8 @@
 
 #define streq(a, b)	      (strcmp (a, b) == 0)
 
+#define END_OF_INSN '\0'
+
 static aarch64_feature_set cpu_variant;
 
 /* Variables that we set while parsing command-line options.  Once all
@@ -5301,6 +5303,41 @@ failure:
 
       if (! backtrack_pos)
 	goto parse_operands_return;
+      else
+	{
+	  /* We reach here because this operand is marked as optional.
+	     For an optional operand, user may have given an operand
+	     instead of using the default value and the given operand
+	     may itself be in wrong format. So although "backtrace_pos"
+	     be true here, it may still caused by operand format error.
+	     we need to identify this.  */
+	  char *tmp = backtrack_pos;
+	  char endchar = END_OF_INSN;
+
+	  if (i != (aarch64_num_of_operands (opcode) - 1))
+	    endchar = ',';
+	  skip_past_char (&tmp, ',');
+
+	  /* User have given optional operand, but the operand is in
+	     wrong format.  */
+	  if (*tmp != endchar)
+	    goto parse_operands_return;
+
+	  /* Try to use the default value, but before that, make sure there is no
+	     extra ',' which is wrong format.  For example, the fifth operand of
+	     'sys' is optional:
+
+	       sys #0,c0,c0,#0,  <--- wrong
+	       sys #0,c0,c0,#0   <--- correct
+
+	       The second format is correct.  */
+	  else if (comma_skipped_p && i && endchar == END_OF_INSN)
+	    {
+	      set_fatal_syntax_error
+		(_("unexpected comma before the omitted optional operand"));
+	      goto parse_operands_return;
+	    }
+	}
 
       /* Reaching here means we are dealing with an optional operand that is
 	 omitted from the assembly line.  */
@@ -5312,15 +5349,6 @@ failure:
       str = backtrack_pos;
       backtrack_pos = 0;
 
-      /* If this is the last operand that is optional and omitted, but without
-	 the presence of a comma.  */
-      if (i && comma_skipped_p && i == aarch64_num_of_operands (opcode) - 1)
-	{
-	  set_fatal_syntax_error
-	    (_("unexpected comma before the omitted optional operand"));
-	  goto parse_operands_return;
-	}
-
       /* Clear any error record after the omitted optional operand has been
 	 successfully handled.  */
       clear_error ();
diff --git a/gas/testsuite/gas/aarch64/diagnostic.l b/gas/testsuite/gas/aarch64/diagnostic.l
index 00a0d7a..d906ce3 100644
--- a/gas/testsuite/gas/aarch64/diagnostic.l
+++ b/gas/testsuite/gas/aarch64/diagnostic.l
@@ -14,7 +14,7 @@
 [^:]*:16: Error: immediate value out of range 0 to 31 at operand 2 -- `ccmp x0,-1,10,le'
 [^:]*:17: Error: extraneous register at operand 2 -- `tlbi alle3is,x0'
 [^:]*:18: Error: missing register at operand 2 -- `tlbi vaale1is'
-[^:]*:19: Error: unexpected characters following instruction at operand 1 -- `tlbi vaale1is x0'
+[^:]*:19: Error: comma expected between operands at operand 2 -- `tlbi vaale1is x0'
 [^:]*:20: Error: immediate value out of range 0 to 1 at operand 1 -- `msr spsel,3'
 [^:]*:21: Error: immediate value out of range 1 to 64 at operand 3 -- `fcvtzu x15,d31,#66'
 [^:]*:22: Error: immediate value out of range 1 to 32 at operand 3 -- `scvtf s0,w0,33'
@@ -97,3 +97,10 @@
 [^:]*:99: Error: operand 3 should be one of the standard conditions, excluding AL and NV. -- `cinc w0,w1,nv'
 [^:]*:100: Error: operand 2 should be one of the standard conditions, excluding AL and NV. -- `cset w0,al'
 [^:]*:101: Error: operand 2 should be one of the standard conditions, excluding AL and NV. -- `cset w0,nv'
+[^:]*:104: Error: operand 1 should be an integer register -- `ret lr'
+[^:]*:105: Error: operand 1 should be an integer register -- `ret kk'
+[^:]*:106: Error: immediate operand required at operand 1 -- `clrex x0'
+[^:]*:107: Error: immediate operand required at operand 1 -- `clrex w0'
+[^:]*:108: Error: constant expression required at operand 1 -- `clrex kk'
+[^:]*:109: Error: operand 5 should be an integer register -- `sys #0,c0,c0,#0,kk'
+[^:]*:110: Error: operand 5 should be an integer register -- `sys #0,c0,c0,#0,lr'
diff --git a/gas/testsuite/gas/aarch64/diagnostic.s b/gas/testsuite/gas/aarch64/diagnostic.s
index 2bb16b0..4210530 100644
--- a/gas/testsuite/gas/aarch64/diagnostic.s
+++ b/gas/testsuite/gas/aarch64/diagnostic.s
@@ -99,3 +99,12 @@
 	cinc	w0, w1, nv
 	cset	w0, al
 	cset	w0, nv
+
+	# test diagnostic info on optional operand
+	ret	lr
+	ret	kk
+	clrex	x0
+	clrex	w0
+	clrex	kk
+	sys	#0, c0, c0, #0, kk
+	sys	#0, c0, c0, #0, lr