diff --git a/msg_helpers_test.go b/msg_helpers_test.go
new file mode 100644
index 00000000..adceb882
--- /dev/null
+++ b/msg_helpers_test.go
@@ -0,0 +1,108 @@
+package dns
+
+import "testing"
+
+// TestPacketDataNsec tests generated using fuzz.go and with a message pack
+// containing the following bytes: 0000\x00\x00000000\x00\x002000000\x0060000\x00\x130000000000000000000"
+// That bytes sequence created the overflow error and further permutations of that sequence were able to trigger
+// the other code paths.
+func TestPackDataNsec(t *testing.T) {
+	type args struct {
+		bitmap []uint16
+		msg    []byte
+		off    int
+	}
+	tests := []struct {
+		name       string
+		args       args
+		want       int
+		wantErr    bool
+		wantErrMsg string
+	}{
+		{
+			name: "overflow",
+			args: args{
+				bitmap: []uint16{
+					8962, 8963, 8970, 8971, 8978, 8979,
+					8986, 8987, 8994, 8995, 9002, 9003,
+					9010, 9011, 9018, 9019, 9026, 9027,
+					9034, 9035, 9042, 9043, 9050, 9051,
+					9058, 9059, 9066,
+				},
+				msg: []byte{
+					48, 48, 48, 48, 0, 0, 0,
+					1, 0, 0, 0, 0, 0, 0, 50,
+					48, 48, 48, 48, 48, 48,
+					0, 54, 48, 48, 48, 48,
+					0, 19, 48, 48,
+				},
+				off: 48,
+			},
+			wantErr:    true,
+			wantErrMsg: "dns: overflow packing nsec",
+			want:       31,
+		},
+		{
+			name: "disordered nsec bits",
+			args: args{
+				bitmap: []uint16{
+					8962,
+					0,
+				},
+				msg: []byte{
+					48, 48, 48, 48, 0, 0, 0, 1, 0, 0, 0, 0,
+					0, 0, 50, 48, 48, 48, 48, 48, 48, 0, 54, 48,
+					48, 48, 48, 0, 19, 48, 48, 48, 48, 48, 48, 0,
+					0, 0, 1, 0, 0, 0, 0, 0, 0, 50, 48, 48,
+					48, 48, 48, 48, 0, 54, 48, 48, 48, 48, 0, 19,
+					48, 48, 48, 48, 48, 48, 0, 0, 0, 1, 0, 0,
+					0, 0, 0, 0, 50, 48, 48, 48, 48, 48, 48, 0,
+					54, 48, 48, 48, 48, 0, 19, 48, 48, 48, 48, 48,
+					48, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 50,
+					48, 48, 48, 48, 48, 48, 0, 54, 48, 48, 48, 48,
+					0, 19, 48, 48, 48, 48, 48, 48, 0, 0, 0, 1,
+					0, 0, 0, 0, 0, 0, 50, 48, 48, 48, 48, 48,
+					48, 0, 54, 48, 48, 48, 48, 0, 19, 48, 48,
+				},
+				off: 0,
+			},
+			wantErr:    true,
+			wantErrMsg: "dns: nsec bits out of order",
+			want:       155,
+		},
+		{
+			name: "simple message with only one window",
+			args: args{
+				bitmap: []uint16{
+					0,
+				},
+				msg: []byte{
+					48, 48, 48, 48, 0, 0,
+					0, 1, 0, 0, 0, 0,
+					0, 0, 50, 48, 48, 48,
+					48, 48, 48, 0, 54, 48,
+					48, 48, 48, 0, 19, 48, 48,
+				},
+				off: 0,
+			},
+			wantErr: false,
+			want:    3,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := packDataNsec(tt.args.bitmap, tt.args.msg, tt.args.off)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("packDataNsec() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if err != nil && tt.wantErrMsg != err.Error() {
+				t.Errorf("packDataNsec() error msg = %v, wantErrMsg %v", err.Error(), tt.wantErrMsg)
+				return
+			}
+			if got != tt.want {
+				t.Errorf("packDataNsec() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}