diff --git a/gas/testsuite/gas/aarch64/illegal.l b/gas/testsuite/gas/aarch64/illegal.l
index 65bd38afd7a..ae9bb939728 100644
--- a/gas/testsuite/gas/aarch64/illegal.l
+++ b/gas/testsuite/gas/aarch64/illegal.l
@@ -879,4 +879,8 @@
 [^:]*:593: Error: .*`st2 {v0\.16b-v1\.16b}\[1\],\[x0\]'
 [^:]*:594: Error: .*`st3 {v0\.16b-v2\.16b}\[2\],\[x0\]'
 [^:]*:595: Error: .*`st4 {v0\.8b-v3\.8b}\[4\],\[x0\]'
-[^:]*:597: Error: .*
+[^:]*:597: Error: the register-index form of PRFM does not accept opcodes in the range 24-31 at operand 1 -- `prfm #0x18,\[sp,x15,lsl#0\]'
+[^:]*:598: Error: the register-index form of PRFM does not accept opcodes in the range 24-31 at operand 1 -- `prfm #0x1f,\[sp,x15,lsl#0\]'
+[^:]*:599: Error: immediate value out of range 0 to 31 at operand 1 -- `prfm #0x20,\[sp,x15,lsl#0\]'
+[^:]*:600: Error: immediate value out of range 0 to 31 at operand 1 -- `prfm #0x20,FOO'
+[^:]*:602: Error: .*
diff --git a/gas/testsuite/gas/aarch64/illegal.s b/gas/testsuite/gas/aarch64/illegal.s
index 384b673e8fb..6fb637ab923 100644
--- a/gas/testsuite/gas/aarch64/illegal.s
+++ b/gas/testsuite/gas/aarch64/illegal.s
@@ -594,4 +594,9 @@ one_label:
 	st3 {v0.16b-v2.16b}[2],[x0]
 	st4 {v0.8b-v3.8b}[4],[x0]
 
+	prfm	#0x18, [sp, x15, lsl #0]
+	prfm	#0x1f, [sp, x15, lsl #0]
+	prfm	#0x20, [sp, x15, lsl #0]
+	prfm	#0x20, FOO
+
 	// End (for errors during literal pool generation)
diff --git a/gas/testsuite/gas/aarch64/system.d b/gas/testsuite/gas/aarch64/system.d
index 93c84a72982..7e4bafbf1ff 100644
--- a/gas/testsuite/gas/aarch64/system.d
+++ b/gas/testsuite/gas/aarch64/system.d
@@ -330,43 +330,27 @@ Disassembly of section \.text:
 .*:	f9800c77 	prfm	#0x17, \[x3, #24\]
 .*:	d8000018 	prfm	#0x18, 0 <LABEL1>
 .*: R_AARCH64_(P32_|)LD_PREL_LO19	LABEL1
-.*:	f8af6bf8 	prfm	#0x18, \[sp, x15\]
-.*:	f8be58f8 	prfm	#0x18, \[x7, w30, uxtw #3\]
 .*:	f9800c78 	prfm	#0x18, \[x3, #24\]
 .*:	d8000019 	prfm	#0x19, 0 <LABEL1>
 .*: R_AARCH64_(P32_|)LD_PREL_LO19	LABEL1
-.*:	f8af6bf9 	prfm	#0x19, \[sp, x15\]
-.*:	f8be58f9 	prfm	#0x19, \[x7, w30, uxtw #3\]
 .*:	f9800c79 	prfm	#0x19, \[x3, #24\]
 .*:	d800001a 	prfm	#0x1a, 0 <LABEL1>
 .*: R_AARCH64_(P32_|)LD_PREL_LO19	LABEL1
-.*:	f8af6bfa 	prfm	#0x1a, \[sp, x15\]
-.*:	f8be58fa 	prfm	#0x1a, \[x7, w30, uxtw #3\]
 .*:	f9800c7a 	prfm	#0x1a, \[x3, #24\]
 .*:	d800001b 	prfm	#0x1b, 0 <LABEL1>
 .*: R_AARCH64_(P32_|)LD_PREL_LO19	LABEL1
-.*:	f8af6bfb 	prfm	#0x1b, \[sp, x15\]
-.*:	f8be58fb 	prfm	#0x1b, \[x7, w30, uxtw #3\]
 .*:	f9800c7b 	prfm	#0x1b, \[x3, #24\]
 .*:	d800001c 	prfm	#0x1c, 0 <LABEL1>
 .*: R_AARCH64_(P32_|)LD_PREL_LO19	LABEL1
-.*:	f8af6bfc 	prfm	#0x1c, \[sp, x15\]
-.*:	f8be58fc 	prfm	#0x1c, \[x7, w30, uxtw #3\]
 .*:	f9800c7c 	prfm	#0x1c, \[x3, #24\]
 .*:	d800001d 	prfm	#0x1d, 0 <LABEL1>
 .*: R_AARCH64_(P32_|)LD_PREL_LO19	LABEL1
-.*:	f8af6bfd 	prfm	#0x1d, \[sp, x15\]
-.*:	f8be58fd 	prfm	#0x1d, \[x7, w30, uxtw #3\]
 .*:	f9800c7d 	prfm	#0x1d, \[x3, #24\]
 .*:	d800001e 	prfm	#0x1e, 0 <LABEL1>
 .*: R_AARCH64_(P32_|)LD_PREL_LO19	LABEL1
-.*:	f8af6bfe 	prfm	#0x1e, \[sp, x15\]
-.*:	f8be58fe 	prfm	#0x1e, \[x7, w30, uxtw #3\]
 .*:	f9800c7e 	prfm	#0x1e, \[x3, #24\]
 .*:	d800001f 	prfm	#0x1f, 0 <LABEL1>
 .*: R_AARCH64_(P32_|)LD_PREL_LO19	LABEL1
-.*:	f8af6bff 	prfm	#0x1f, \[sp, x15\]
-.*:	f8be58ff 	prfm	#0x1f, \[x7, w30, uxtw #3\]
 .*:	f9800c7f 	prfm	#0x1f, \[x3, #24\]
 .*:	f9800c60 	prfm	pldl1keep, \[x3, #24\]
 .*:	f9800c61 	prfm	pldl1strm, \[x3, #24\]
@@ -386,3 +370,5 @@ Disassembly of section \.text:
 .*:	f9800c73 	prfm	pstl2strm, \[x3, #24\]
 .*:	f9800c74 	prfm	pstl3keep, \[x3, #24\]
 .*:	f9800c75 	prfm	pstl3strm, \[x3, #24\]
+.*:	f8a04817 	prfm	#0x17, \[x0, w0, uxtw\]
+.*:	f8a04818 	\.inst	0xf8a04818 ; undefined
diff --git a/gas/testsuite/gas/aarch64/system.s b/gas/testsuite/gas/aarch64/system.s
index 4d24d9a7614..48e7bfeb103 100644
--- a/gas/testsuite/gas/aarch64/system.s
+++ b/gas/testsuite/gas/aarch64/system.s
@@ -70,8 +70,10 @@
 
 	.macro	all_prefetchs op, from=0, to=31
 	\op	\from, LABEL1
+	.if	\from < 24
 	\op	\from, [sp, x15, lsl #0]
 	\op	\from, [x7, w30, uxtw #3]
+	.endif
 	\op	\from, [x3, #24]
 	.if	\to-\from
 	all_prefetchs \op, "(\from+1)", \to
@@ -91,3 +93,6 @@
 	.endr
 	.endr
 	.endr
+
+	.inst	0xf8a04817
+	.inst	0xf8a04818
diff --git a/opcodes/aarch64-opc.c b/opcodes/aarch64-opc.c
index e271b0d5e8e..a0e6240592c 100644
--- a/opcodes/aarch64-opc.c
+++ b/opcodes/aarch64-opc.c
@@ -2599,6 +2599,15 @@ operand_general_constraint_met_p (const aarch64_opnd_info *opnds, int idx,
 	      return 0;
 	    }
 	  break;
+	case AARCH64_OPND_PRFOP:
+	  if (opcode->iclass == ldst_regoff && opnd->prfop->value >= 24)
+	    {
+	      set_other_error (mismatch_detail, idx,
+			       _("the register-index form of PRFM does"
+				 " not accept opcodes in the range 24-31"));
+	      return 0;
+	    }
+	  break;
 	default:
 	  break;
 	}
