diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
index 3fc86ab825..bae987cf66 100644
--- a/gdb/riscv-tdep.c
+++ b/gdb/riscv-tdep.c
@@ -1385,10 +1385,12 @@ riscv_insn::decode (struct gdbarch *gdbarch, CORE_ADDR pc)
 	m_opcode = OTHER;
     }
   else
-    internal_error (__FILE__, __LINE__,
-		    _("unable to decode %d byte instructions in "
-		      "prologue at %s"), m_length,
-		    core_addr_to_string (pc));
+    {
+      /* This must be a 6 or 8 byte instruction, we don't currently decode
+	 any of these, so just ignore it.  */
+      gdb_assert (m_length == 6 || m_length == 8);
+      m_opcode = OTHER;
+    }
 }
 
 /* The prologue scanner.  This is currently only used for skipping the
diff --git a/gdb/testsuite/gdb.arch/riscv-unwind-long-insn-6.s b/gdb/testsuite/gdb.arch/riscv-unwind-long-insn-6.s
new file mode 100644
index 0000000000..b21b1e10f3
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/riscv-unwind-long-insn-6.s
@@ -0,0 +1,45 @@
+/* Copyright 2019 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+	.option nopic
+	.text
+
+        .align	1
+	.globl	bar
+	.type	bar, @function
+bar:
+        tail       1f
+	.size	bar, .-func
+
+	.align	1
+	.globl	func
+	.type	func, @function
+func:
+        /* A fake 6 byte instruction.  This is never executed, but the
+	   prologue scanner will try to decode it.  These long
+	   instructions are ISA extensions, I use .byte rather than an
+	   actual instruction mnemonic so that the test can be compiled
+	   with a toolchain that doesn't include any long instruction
+	   extensions.  */
+        .byte 0x1f, 0x00, 0x00, 0x00, 0x00, 0x00
+1:
+	addi	sp,sp,-16
+	sw	s0,12(sp)
+	addi	s0,sp,16
+	nop
+	lw	s0,12(sp)
+	addi	sp,sp,16
+	jr	ra
+	.size	func, .-func
diff --git a/gdb/testsuite/gdb.arch/riscv-unwind-long-insn-8.s b/gdb/testsuite/gdb.arch/riscv-unwind-long-insn-8.s
new file mode 100644
index 0000000000..3fad07b59d
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/riscv-unwind-long-insn-8.s
@@ -0,0 +1,45 @@
+/* Copyright 2019 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+	.option nopic
+	.text
+
+        .align	1
+	.globl	bar
+	.type	bar, @function
+bar:
+        tail       1f
+	.size	bar, .-func
+
+	.align	1
+	.globl	func
+	.type	func, @function
+func:
+        /* A fake 8 byte instruction.  This is never executed, but the
+	   prologue scanner will try to decode it.  These long
+	   instructions are ISA extensions, I use .byte rather than an
+	   actual instruction mnemonic so that the test can be compiled
+	   with a toolchain that doesn't include any long instruction
+	   extensions.  */
+        .byte 0x3f, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
+1:
+	addi	sp,sp,-16
+	sw	s0,12(sp)
+	addi	s0,sp,16
+	nop
+	lw	s0,12(sp)
+	addi	sp,sp,16
+	jr	ra
+	.size	func, .-func
diff --git a/gdb/testsuite/gdb.arch/riscv-unwind-long-insn.c b/gdb/testsuite/gdb.arch/riscv-unwind-long-insn.c
new file mode 100644
index 0000000000..d601e2d3d8
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/riscv-unwind-long-insn.c
@@ -0,0 +1,25 @@
+/* Copyright 2019 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+extern void func (void);
+extern void bar (void);
+
+int
+main ()
+{
+  bar ();
+  func ();
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.arch/riscv-unwind-long-insn.exp b/gdb/testsuite/gdb.arch/riscv-unwind-long-insn.exp
new file mode 100644
index 0000000000..ddb914295c
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/riscv-unwind-long-insn.exp
@@ -0,0 +1,61 @@
+# Copyright 2019 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This tests GDB's ability to handle 6 and 8 byte instructions in the
+# RISC-V prologue scanner.  These instruction should be ignored, but
+# should not result in an error that interrupts the debug session.
+#
+# Each of the files riscv-unwind-long-insn-*.s include a function
+# (func) that contains a fake long instruction (6 or 8 bytes) in the
+# prologue.  We trick GDB into parsing the fake instruction by tail
+# calling from a different function, 'bar' to the middle of 'func'.
+
+if {![istarget "riscv*-*-*"]} {
+    verbose "Skipping ${gdb_test_file_name}."
+    return
+}
+
+foreach_with_prefix {insn_size} {6 8} {
+    standard_testfile riscv-unwind-long-insn.c \
+	riscv-unwind-long-insn-${insn_size}.s
+
+    set testfile "${testfile}-${insn_size}"
+    if {[prepare_for_testing "failed to prepare" $testfile \
+	     "$srcfile $srcfile2"  debug]} {
+	return -1
+    }
+
+    if ![runto_main] then {
+	fail "can't run to main"
+	return 0
+    }
+
+    gdb_breakpoint "bar"
+    gdb_continue_to_breakpoint "bar"
+
+    # This next single instruction step takes us through a tail-call
+    # from 'bar' into 'func'.
+    gdb_test "si" "func \(\).*"
+
+    # Now check that we have a sane backtrace.
+    gdb_test "bt" \
+	[multi_line \
+	     "#0\[ \t\]*func \\\(\\\) at .*$srcfile2:\[0-9\]+" \
+	     "#1\[ \t\]*$hex in main \\\(\\\) at .*$srcfile:\[0-9\]+"] \
+	"Backtrace to the main frame"
+
+    # Finally finish, and we should end up back in main.
+    gdb_test "finish" "main \\\(\\\) at .*$srcfile:.*"
+}
