exception

java.lang.RuntimeException: You are not attached to this Computer
    at dan200.computercraft.core.apis.PeripheralAPI$PeripheralWrapper.getAttachmentName(PeripheralAPI.java:230)
    at testmod.TestPeripheral.callMethod(TestPeripheral.java:23)
    at dan200.computercraft.core.apis.PeripheralAPI$PeripheralWrapper.call(PeripheralAPI.java:110)
    at dan200.computercraft.core.apis.PeripheralAPI.callMethod(PeripheralAPI.java:477)
    at dan200.computercraft.core.lua.LuaJLuaMachine$2.invoke(LuaJLuaMachine.java:332)
    at org.luaj.vm2.lib.VarArgFunction.onInvoke(Unknown Source)
    at org.luaj.vm2.TailcallVarargs.eval(Unknown Source)
    at org.luaj.vm2.TailcallVarargs.arg1(Unknown Source)
    at org.luaj.vm2.LuaClosure.call(Unknown Source)
    at org.luaj.vm2.LuaClosure.execute(Unknown Source)
    at org.luaj.vm2.LuaClosure.onInvoke(Unknown Source)
    at org.luaj.vm2.LuaClosure.invoke(Unknown Source)
    at org.luaj.vm2.LuaClosure.execute(Unknown Source)
    at org.luaj.vm2.LuaClosure.onInvoke(Unknown Source)
    at org.luaj.vm2.LuaClosure.invoke(Unknown Source)
    at org.luaj.vm2.lib.BaseLib.pcall(Unknown Source)
    at org.luaj.vm2.lib.BaseLib$BaseLibV.invoke(Unknown Source)
    at org.luaj.vm2.LuaClosure.execute(Unknown Source)
    at org.luaj.vm2.LuaClosure.onInvoke(Unknown Source)
    at org.luaj.vm2.LuaClosure.invoke(Unknown Source)
    at org.luaj.vm2.LuaClosure.execute(Unknown Source)
    at org.luaj.vm2.LuaClosure.onInvoke(Unknown Source)
    at org.luaj.vm2.TailcallVarargs.eval(Unknown Source)
    at org.luaj.vm2.TailcallVarargs.arg1(Unknown Source)
    at org.luaj.vm2.LuaClosure.call(Unknown Source)
    at org.luaj.vm2.LuaClosure.execute(Unknown Source)
    at org.luaj.vm2.LuaClosure.onInvoke(Unknown Source)
    at org.luaj.vm2.LuaClosure.invoke(Unknown Source)
    at org.luaj.vm2.LuaClosure.execute(Unknown Source)
    at org.luaj.vm2.LuaClosure.onInvoke(Unknown Source)
    at org.luaj.vm2.LuaClosure.invoke(Unknown Source)
    at org.luaj.vm2.lib.BaseLib.pcall(Unknown Source)
    at org.luaj.vm2.lib.BaseLib$BaseLibV.invoke(Unknown Source)
    at org.luaj.vm2.LuaClosure.execute(Unknown Source)
    at org.luaj.vm2.LuaClosure.call(Unknown Source)
    at org.luaj.vm2.LuaClosure.execute(Unknown Source)
    at org.luaj.vm2.LuaClosure.onInvoke(Unknown Source)
    at org.luaj.vm2.LuaClosure.invoke(Unknown Source)
    at org.luaj.vm2.LuaThread$State.run(Unknown Source)
    at java.lang.Thread.run(Thread.java:744)

test.lua

while true do peripheral.call("left", "testAttached") sleep(0) end

TestPeripheral.java

package testmod;

import dan200.computercraft.api.lua.ILuaContext;
import dan200.computercraft.api.lua.LuaException;
import dan200.computercraft.api.peripheral.IComputerAccess;
import dan200.computercraft.api.peripheral.IPeripheral;

public class TestPeripheral implements IPeripheral {

	@Override
	public String getType() {
		return "faulty_peripheral";
	}

	@Override
	public String[] getMethodNames() {
		return new String[] { "testAttached" };
	}

	@Override
	public Object[] callMethod(IComputerAccess computer, ILuaContext context, int method, Object[] arguments) throws LuaException {
		try {
			return new Object[] { computer.getAttachmentName() };
		} catch (Exception e) {
			e.printStackTrace();
			throw new LuaException("fail");
		}
	}

	@Override
	public void attach(IComputerAccess computer) {}

	@Override
	public void detach(IComputerAccess computer) {}

	@Override
	public boolean equals(IPeripheral other) {
		return other == this;
	}
}

TestProvider.java

package testmod;

import net.minecraft.block.Block;
import net.minecraft.init.Blocks;
import net.minecraft.world.World;
import dan200.computercraft.api.peripheral.IPeripheral;
import dan200.computercraft.api.peripheral.IPeripheralProvider;

public class TestProvider implements IPeripheralProvider {

	@Override
	public IPeripheral getPeripheral(World world, int x, int y, int z, int side) {
		Block block = world.getBlock(x, y, z);
		return (block == Blocks.gold_block) ? new TestPeripheral() : null;
	}

}