diff --git a/src/oemof/solph/flows/_simple_flow_block.py b/src/oemof/solph/flows/_simple_flow_block.py
index 7b0e0a148..469ec1311 100644
--- a/src/oemof/solph/flows/_simple_flow_block.py
+++ b/src/oemof/solph/flows/_simple_flow_block.py
@@ -255,7 +255,7 @@ def _positive_gradient_flow_rule(model):
             """Rule definition for positive gradient constraint."""
             for inp, out in self.POSITIVE_GRADIENT_FLOWS:
                 for index in range(1, len(m.TIMESTEPS) + 1):
-                    if m.TIMESTEPS.at(index)[1] > 0:
+                    if m.TIMESTEPS.at(index) > 0:
                         lhs = (
                             m.flow[
                                 inp,
@@ -269,7 +269,7 @@ def _positive_gradient_flow_rule(model):
                             ]
                         )
                         rhs = self.positive_gradient[
-                            inp, out, m.TIMESTEPS.at(index)[1]
+                            inp, out, m.TIMESTEPS.at(index)
                         ]
                         self.positive_gradient_constr.add(
                             (inp, out, m.TIMESTEPS.at(index)),
diff --git a/tests/lp_files/source_with_gradient.lp b/tests/lp_files/source_with_gradient.lp
index 75f93f31b..c65753b4c 100644
--- a/tests/lp_files/source_with_gradient.lp
+++ b/tests/lp_files/source_with_gradient.lp
@@ -2,60 +2,60 @@
 
 min 
 objective:
-+23 flow(powerplant_electricityBus_0_0)
-+23 flow(powerplant_electricityBus_0_1)
-+23 flow(powerplant_electricityBus_0_2)
++23 flow(powerplant_electricityBus_0)
++23 flow(powerplant_electricityBus_1)
++23 flow(powerplant_electricityBus_2)
 
 s.t.
 
-c_e_BusBlock_balance(electricityBus_0_0)_:
-+1 flow(powerplant_electricityBus_0_0)
+c_e_BusBlock_balance(electricityBus_0)_:
++1 flow(powerplant_electricityBus_0)
 = 0
 
-c_e_BusBlock_balance(electricityBus_0_1)_:
-+1 flow(powerplant_electricityBus_0_1)
+c_e_BusBlock_balance(electricityBus_1)_:
++1 flow(powerplant_electricityBus_1)
 = 0
 
-c_e_BusBlock_balance(electricityBus_0_2)_:
-+1 flow(powerplant_electricityBus_0_2)
+c_e_BusBlock_balance(electricityBus_2)_:
++1 flow(powerplant_electricityBus_2)
 = 0
 
-c_e_SimpleFlowBlock_positive_gradient_constr(powerplant_electricityBus_0_0)_:
+c_e_SimpleFlowBlock_positive_gradient_constr(powerplant_electricityBus_0)_:
 +1 SimpleFlowBlock_positive_gradient(powerplant_electricityBus_0)
 = 0
 
-c_u_SimpleFlowBlock_positive_gradient_constr(powerplant_electricityBus_0_1)_:
--1 flow(powerplant_electricityBus_0_0)
-+1 flow(powerplant_electricityBus_0_1)
+c_u_SimpleFlowBlock_positive_gradient_constr(powerplant_electricityBus_1)_:
+-1 flow(powerplant_electricityBus_0)
++1 flow(powerplant_electricityBus_1)
 -1 SimpleFlowBlock_positive_gradient(powerplant_electricityBus_1)
 <= 0
 
-c_u_SimpleFlowBlock_positive_gradient_constr(powerplant_electricityBus_0_2)_:
--1 flow(powerplant_electricityBus_0_1)
-+1 flow(powerplant_electricityBus_0_2)
+c_u_SimpleFlowBlock_positive_gradient_constr(powerplant_electricityBus_2)_:
+-1 flow(powerplant_electricityBus_1)
++1 flow(powerplant_electricityBus_2)
 -1 SimpleFlowBlock_positive_gradient(powerplant_electricityBus_2)
 <= 0
 
-c_e_SimpleFlowBlock_negative_gradient_constr(powerplant_electricityBus_0_0)_:
+c_e_SimpleFlowBlock_negative_gradient_constr(powerplant_electricityBus_0)_:
 +1 SimpleFlowBlock_negative_gradient(powerplant_electricityBus_0)
 = 0
 
-c_u_SimpleFlowBlock_negative_gradient_constr(powerplant_electricityBus_0_1)_:
-+1 flow(powerplant_electricityBus_0_0)
--1 flow(powerplant_electricityBus_0_1)
+c_u_SimpleFlowBlock_negative_gradient_constr(powerplant_electricityBus_1)_:
++1 flow(powerplant_electricityBus_0)
+-1 flow(powerplant_electricityBus_1)
 -1 SimpleFlowBlock_negative_gradient(powerplant_electricityBus_1)
 <= 0
 
-c_u_SimpleFlowBlock_negative_gradient_constr(powerplant_electricityBus_0_2)_:
-+1 flow(powerplant_electricityBus_0_1)
--1 flow(powerplant_electricityBus_0_2)
+c_u_SimpleFlowBlock_negative_gradient_constr(powerplant_electricityBus_2)_:
++1 flow(powerplant_electricityBus_1)
+-1 flow(powerplant_electricityBus_2)
 -1 SimpleFlowBlock_negative_gradient(powerplant_electricityBus_2)
 <= 0
 
 bounds
-   0 <= flow(powerplant_electricityBus_0_0) <= 999
-   0 <= flow(powerplant_electricityBus_0_1) <= 999
-   0 <= flow(powerplant_electricityBus_0_2) <= 999
+   0 <= flow(powerplant_electricityBus_0) <= 999
+   0 <= flow(powerplant_electricityBus_1) <= 999
+   0 <= flow(powerplant_electricityBus_2) <= 999
    0 <= SimpleFlowBlock_positive_gradient(powerplant_electricityBus_0) <= 29.97
    0 <= SimpleFlowBlock_positive_gradient(powerplant_electricityBus_1) <= 29.97
    0 <= SimpleFlowBlock_positive_gradient(powerplant_electricityBus_2) <= 29.97