diff --git a/eth/vm/logic/flow.py b/eth/vm/logic/flow.py
index 5fcd633e35..fcf7527b26 100644
--- a/eth/vm/logic/flow.py
+++ b/eth/vm/logic/flow.py
@@ -1,6 +1,7 @@
 from eth.exceptions import (
     InvalidJumpDestination,
     InvalidInstruction,
+    OutOfGas,
     Halt,
 )
 
@@ -61,33 +62,29 @@ def gas(computation: BaseComputation) -> None:
 
 
 def beginsub(computation: BaseComputation) -> None:
-    #TODO: raise OOG exception
-    pass
+    raise OutOfGas
 
 
 def jumpsub(computation: BaseComputation) -> None:
     sub_loc = computation.stack_pop1_int()
 
-    temp = computation.code.program_counter
-    computation.code.program_counter = sub_loc
-
-    next_opcode = computation.code.peek()
-
-    if next_opcode != BEGINSUB:
-        #TODO: abort execution
-        pass
-    
-    else:
-        computation.code.program_counter += 1
+    if computation.code.is_valid_opcode(sub_loc):
     
-    computation.rstack_push_int(temp + 1)
+        sub_op = computation.code(sub_loc)
+
+        if sub_op == BEGINSUB:
+            temp = computation.code.program_counter
+            computation.code.program_counter = sub_loc + 1
+            computation.rstack_push_int(temp + 1)
 
 
 def returnsub(computation: BaseComputation) -> None:
 
-    if computation.rstack.length == 0:
-        #TODO: abort execution
+    try:
+        ret_loc = computation.rstack_pop1_int()
+    except InsufficientStack:
         pass
+        
     
-    computation.code.program_counter = computation.rstack_pop1_int()
+    computation.code.program_counter = ret_loc