diff --git a/gas/config/tc-aarch64.c b/gas/config/tc-aarch64.c
index ecb15d23437..aad0c4fb6f3 100644
--- a/gas/config/tc-aarch64.c
+++ b/gas/config/tc-aarch64.c
@@ -4106,11 +4106,13 @@ parse_sys_reg (char **str, struct hash_control *sys_regs,
 
   p = buf;
   for (q = *str; ISALNUM (*q) || *q == '_'; q++)
-    if (p < buf + 31)
+    if (p < buf + (sizeof (buf) - 1))
       *p++ = TOLOWER (*q);
   *p = '\0';
-  /* Assert that BUF be large enough.  */
-  gas_assert (p - buf == q - *str);
+
+  /* Assume that our buffer is big enough for any valid system register.  */
+  if (p - buf != q - *str)
+    return PARSE_FAIL;
 
   o = hash_find (sys_regs, buf);
   if (!o)
diff --git a/gas/testsuite/gas/aarch64/invalid-sysreg-assert.d b/gas/testsuite/gas/aarch64/invalid-sysreg-assert.d
new file mode 100644
index 00000000000..a6279bbe7c2
--- /dev/null
+++ b/gas/testsuite/gas/aarch64/invalid-sysreg-assert.d
@@ -0,0 +1,3 @@
+#name: don't assert on long system register
+#source: invalid-sysreg-assert.s
+#error_output: invalid-sysreg-assert.l
diff --git a/gas/testsuite/gas/aarch64/invalid-sysreg-assert.l b/gas/testsuite/gas/aarch64/invalid-sysreg-assert.l
new file mode 100644
index 00000000000..b6049108b07
--- /dev/null
+++ b/gas/testsuite/gas/aarch64/invalid-sysreg-assert.l
@@ -0,0 +1,2 @@
+[^:]*: Assembler messages:
+.*: Error: unknown or missing system register name at operand 1 -- `msr 00000000000000000000000000000000'
diff --git a/gas/testsuite/gas/aarch64/invalid-sysreg-assert.s b/gas/testsuite/gas/aarch64/invalid-sysreg-assert.s
new file mode 100644
index 00000000000..8b3706fd9cd
--- /dev/null
+++ b/gas/testsuite/gas/aarch64/invalid-sysreg-assert.s
@@ -0,0 +1,2 @@
+// This input caused an assertion failure in parse_sys_reg.
+msr 00000000000000000000000000000000