KeyError

Hello, 
I am writing the code for the optimization problem, but here is the error message I have:

KeyError: 'L4'

Please help me figure out how to solve it. 

import gurobipy as gp
from gurobipy import GRB

# Data
generating_units = {
    'g1': {'node': 'n1', 'generator_capacity': 300, 'production_cost': 18},
    'g2': {'node': 'n2', 'generator_capacity': 250, 'production_cost': 25},
    'g3': {'node': 'n3', 'generator_capacity': 400, 'production_cost': 16},
    'g4': {'node': 'n4', 'generator_capacity': 300, 'production_cost': 32},
    'g5': {'node': 'n5', 'generator_capacity': 150, 'production_cost': 35}
}

demands = {
    'd1': {'node': 'n1', 'demand': 200, 'shedding_cost': 18},
    'd2': {'node': 'n4', 'demand': 150, 'shedding_cost': 25},
    'd3': {'node': 'n5', 'demand': 100, 'shedding_cost': 16},
    'd4': {'node': 'n6', 'demand': 200, 'shedding_cost': 32}
}

existing_lines = {
    'l1': {'sender': 'n1', 'receiver': 'n2', 'susceptance': 500, 'line_capacity': 150},
    'l2': {'sender': 'n1', 'receiver': 'n3', 'susceptance': 500, 'line_capacity': 150},
    'l3': {'sender': 'n4', 'receiver': 'n5', 'susceptance': 500, 'line_capacity': 150}
}

prospective_lines = {
    'L4': {'sender': 'n2', 'receiver': 'n3', 'susceptance': 500, 'line_capacity': 150, 'line_build_cost': 700000},
    'L5': {'sender': 'n2', 'receiver': 'n4', 'susceptance': 500, 'line_capacity': 200, 'line_build_cost': 1400000},
    'L6': {'sender': 'n3', 'receiver': 'n4', 'susceptance': 500, 'line_capacity': 200, 'line_build_cost': 1800000},
    'L7': {'sender': 'n3', 'receiver': 'n6', 'susceptance': 500, 'line_capacity': 200, 'line_build_cost': 1600000},
    'L8': {'sender': 'n4', 'receiver': 'n6', 'susceptance': 500, 'line_capacity': 150, 'line_build_cost': 800000},
    'L9': {'sender': 'n5', 'receiver': 'n6', 'susceptance': 500, 'line_capacity': 150, 'line_build_cost': 700000}
}

investment_budget = 3000000
sigma = 8760
planning_horizon = 20
base_power = 1
base_voltage = 1


# Create a model
model = gp.Model("TEP")

# Decision variables
build_line = model.addVars(transmission_lines_prospective, vtype=GRB.BINARY, name="Build_Line")
power_generated = model.addVars(generating_units, vtype=GRB.CONTINUOUS, name="Power_Generated")
power_flow_existing = model.addVars(existing_lines, vtype=GRB.CONTINUOUS, name="{Power_flow_Existing}")
power_flow_candidate = model.addVars(prospective_lines, vtype=GRB.CONTINUOUS, name="Power_flow_candidate")
power_demand = model.addVars(demands, vtype=GRB.CONTINUOUS, name="Power_Demand")
load_shedding = model.addVars(demands, vtype=GRB.CONTINUOUS, name="Load_Shedding")
model.update()

#Objective Function 


investment_cost = gp.quicksum(line_build_cost[line] * build_line[line] for line in prospective_lines) 
operating_cost = gp.quicksum(production_cost[generator] * generator_capacity[generator] for generator in generating_units) 
load_shedding_cost =  gp.quicksum(shedding_cost[demand] * demands[demand] for demand in demands)

model.setObjective((investment_cost + sigma(operating_cost + shedding_cost), GRB.MINIMIZE))




# Constraints
# Budget constraint
# model.addConstr(gp.quicksum(prospective_lines[line]['line_build_cost'] * build_line[line] for line in prospective_lines) <= investment_budget)
model.addConstr(gp.quicksum(build_line[line] * prospective_lines[line] for line in prospective_lines) <= investment_budget, "Budget")