diff --git a/core/agent.py b/core/agent.py
index ee9e7ec..984fc35 100644
--- a/core/agent.py
+++ b/core/agent.py
@@ -134,16 +134,16 @@ def _save_model(self, step, curr_reward):
             self.logger.warning("Saved  Model    @ Step: " + str(step) + ": " + self.model_name + ".")
 
     def _forward(self, observation):
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
 
     def _backward(self, reward, terminal):
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
 
     def _eval_model(self):  # evaluation during training
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
 
     def fit_model(self):    # training
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
 
     def test_model(self):   # testing pre-trained models
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
diff --git a/core/agent_single_process.py b/core/agent_single_process.py
index 5a9213a..2352175 100644
--- a/core/agent_single_process.py
+++ b/core/agent_single_process.py
@@ -53,10 +53,10 @@ def _ensure_global_grads(self):
             global_param._grad = local_param.grad
 
     def _forward(self, observation):
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
 
     def _backward(self, reward, terminal):
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
 
     def run(self):
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
diff --git a/core/env.py b/core/env.py
index f203695..a93bea0 100644
--- a/core/env.py
+++ b/core/env.py
@@ -51,11 +51,11 @@ def _get_experience(self):
                           terminal1 = self.exp_terminal1)
 
     def _preprocessState(self, state):
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
 
     @property
     def state_shape(self):
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
 
     @property
     def action_dim(self):
@@ -65,13 +65,13 @@ def action_dim(self):
             return self.env.action_space.n
 
     def render(self):       # render using the original gl window
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
 
     def visual(self):       # visualize onto visdom
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
 
     def reset(self):
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
 
     def step(self, action):
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
diff --git a/core/model.py b/core/model.py
index 507909b..bce1cf4 100644
--- a/core/model.py
+++ b/core/model.py
@@ -31,7 +31,7 @@ def __init__(self, args):
         self.output_dims    = args.action_dim
 
     def _init_weights(self):
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")
 
     def print_model(self):
         self.logger.warning("<-----------------------------------> Model")
@@ -43,4 +43,4 @@ def _reset(self):           # NOTE: should be called at each child's __init__
         self.print_model()
 
     def forward(self, input):
-        raise NotImplementedError("not implemented in base calss")
+        raise NotImplementedError("not implemented in base class")