diff --git a/proto/merge.go b/proto/merge.go
index 16a79d6..df72f98 100644
--- a/proto/merge.go
+++ b/proto/merge.go
@@ -68,6 +68,10 @@
 		}
 	}
 
+	if !dst.IsValid() {
+		panic("cannot merge into invalid destination message")
+	}
+
 	src.Range(func(fd protoreflect.FieldDescriptor, v protoreflect.Value) bool {
 		switch {
 		case fd.IsList():
diff --git a/proto/nil_test.go b/proto/nil_test.go
new file mode 100644
index 0000000..06e76ae
--- /dev/null
+++ b/proto/nil_test.go
@@ -0,0 +1,168 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package proto_test
+
+import (
+	"testing"
+
+	"google.golang.org/protobuf/proto"
+
+	testpb "google.golang.org/protobuf/internal/testprotos/test"
+)
+
+// TestNil tests for boundary conditions when nil and typed-nil messages
+// are passed to various top-level functions.
+// These tests are not necessarily a statement of proper behavior,
+// but exist to detect accidental changes in behavior.
+func TestNil(t *testing.T) {
+	nilMsg := (*testpb.TestAllExtensions)(nil)
+	extType := testpb.E_OptionalBool
+
+	tests := []struct {
+		label string
+		test  func()
+		panic bool
+	}{{
+		label: "Size",
+		test:  func() { proto.Size(nil) },
+		panic: true,
+	}, {
+		label: "Size",
+		test:  func() { proto.Size(nilMsg) },
+	}, {
+		label: "Marshal",
+		test:  func() { proto.Marshal(nil) },
+		panic: true,
+	}, {
+		label: "Marshal",
+		test:  func() { proto.Marshal(nilMsg) },
+	}, {
+		label: "Unmarshal",
+		test:  func() { proto.Unmarshal(nil, nil) },
+		panic: true,
+	}, {
+		label: "Unmarshal",
+		test:  func() { proto.Unmarshal(nil, nilMsg) },
+		panic: true,
+	}, {
+		label: "Merge",
+		test:  func() { proto.Merge(nil, nil) },
+		panic: true,
+	}, {
+		label: "Merge",
+		test:  func() { proto.Merge(nil, nilMsg) },
+		panic: true,
+	}, {
+		label: "Merge",
+		test:  func() { proto.Merge(nilMsg, nil) },
+		panic: true,
+	}, {
+		label: "Merge",
+		test:  func() { proto.Merge(nilMsg, nilMsg) },
+		panic: true,
+	}, {
+		label: "Clone",
+		test:  func() { proto.Clone(nil) },
+	}, {
+		label: "Clone",
+		test:  func() { proto.Clone(nilMsg) },
+	}, {
+		label: "Equal",
+		test:  func() { proto.Equal(nil, nil) },
+	}, {
+		label: "Equal",
+		test:  func() { proto.Equal(nil, nilMsg) },
+	}, {
+		label: "Equal",
+		test:  func() { proto.Equal(nilMsg, nil) },
+	}, {
+		label: "Equal",
+		test:  func() { proto.Equal(nilMsg, nilMsg) },
+	}, {
+		label: "Reset",
+		test:  func() { proto.Reset(nil) },
+		panic: true,
+	}, {
+		label: "Reset",
+		test:  func() { proto.Reset(nilMsg) },
+		panic: true,
+	}, {
+		label: "HasExtension",
+		test:  func() { proto.HasExtension(nil, nil) },
+		panic: true,
+	}, {
+		label: "HasExtension",
+		test:  func() { proto.HasExtension(nil, extType) },
+		panic: true,
+	}, {
+		label: "HasExtension",
+		test:  func() { proto.HasExtension(nilMsg, nil) },
+		panic: true,
+	}, {
+		label: "HasExtension",
+		test:  func() { proto.HasExtension(nilMsg, extType) },
+	}, {
+		label: "GetExtension",
+		test:  func() { proto.GetExtension(nil, nil) },
+		panic: true,
+	}, {
+		label: "GetExtension",
+		test:  func() { proto.GetExtension(nil, extType) },
+		panic: true,
+	}, {
+		label: "GetExtension",
+		test:  func() { proto.GetExtension(nilMsg, nil) },
+		panic: true,
+	}, {
+		label: "GetExtension",
+		test:  func() { proto.GetExtension(nilMsg, extType) },
+	}, {
+		label: "SetExtension",
+		test:  func() { proto.SetExtension(nil, nil, true) },
+		panic: true,
+	}, {
+		label: "SetExtension",
+		test:  func() { proto.SetExtension(nil, extType, true) },
+		panic: true,
+	}, {
+		label: "SetExtension",
+		test:  func() { proto.SetExtension(nilMsg, nil, true) },
+		panic: true,
+	}, {
+		label: "SetExtension",
+		test:  func() { proto.SetExtension(nilMsg, extType, true) },
+		panic: true,
+	}, {
+		label: "ClearExtension",
+		test:  func() { proto.ClearExtension(nil, nil) },
+		panic: true,
+	}, {
+		label: "ClearExtension",
+		test:  func() { proto.ClearExtension(nil, extType) },
+		panic: true,
+	}, {
+		label: "ClearExtension",
+		test:  func() { proto.ClearExtension(nilMsg, nil) },
+		panic: true,
+	}, {
+		label: "ClearExtension",
+		test:  func() { proto.ClearExtension(nilMsg, extType) },
+		panic: true,
+	}}
+
+	for _, tt := range tests {
+		t.Run(tt.label, func(t *testing.T) {
+			defer func() {
+				switch gotPanic := recover() != nil; {
+				case gotPanic && !tt.panic:
+					t.Errorf("unexpected panic")
+				case !gotPanic && tt.panic:
+					t.Errorf("expected panic")
+				}
+			}()
+			tt.test()
+		})
+	}
+}
diff --git a/proto/reset.go b/proto/reset.go
index 57bbc3a..22048bc 100644
--- a/proto/reset.go
+++ b/proto/reset.go
@@ -18,6 +18,10 @@
 }
 
 func resetMessage(m protoreflect.Message) {
+	if !m.IsValid() {
+		panic("cannot reset invalid message")
+	}
+
 	// Clear all known fields.
 	fds := m.Descriptor().Fields()
 	for i := 0; i < fds.Len(); i++ {