diff --git a/src/lib.rs b/src/lib.rs
index b0a33e0..bff3e0d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -40,13 +40,13 @@ impl Stack {
         }
     }
     pub fn print_memory(&self) {
-        if self.mem.len() > 0 {
+        if !self.mem.is_empty() {
             println!("memory ({}):", self.mem.len());
             println!("{:?}", vec_u8_to_hex(self.mem.to_vec()));
         }
     }
     pub fn print_storage(&self) {
-        if self.storage.len() > 0 {
+        if !self.storage.is_empty() {
             println!("storage ({}):", self.storage.len());
             for (key, value) in self.storage.iter() {
                 println!(
@@ -79,9 +79,16 @@ impl Stack {
     pub fn pop(&mut self) -> Result<[u8; 32], String> {
         match self.stack.pop() {
             Some(x) => Ok(x),
-            None => Err(format!("pop err")), // WIP
+            None => Err("pop err".to_string()), // WIP
         }
     }
+    pub fn substract_gas(&mut self, val: u64) -> Result<(), String> {
+        if self.gas < val {
+            return Err("out of gas".to_string());
+        }
+        self.gas -= val;
+        Ok(())
+    }
 
     pub fn execute(
         &mut self,
@@ -150,8 +157,8 @@ impl Stack {
                         0x51 => self.mload()?,
                         0x52 => self.mstore()?,
                         0x55 => self.sstore()?,
-                        0x56 => self.jump()?,
-                        0x57 => self.jump_i()?,
+                        0x56 => self.jump(code)?,
+                        0x57 => self.jump_i(code)?,
                         0x5b => self.jump_dest()?,
                         _ => return Err(format!("unimplemented {:x}", opcode)),
                     }
@@ -195,7 +202,7 @@ impl Stack {
                     return Err(format!("unimplemented {:x}", opcode));
                 }
             }
-            self.gas -= self.opcodes.get(&opcode).unwrap().gas;
+            self.substract_gas(self.opcodes.get(&opcode).unwrap().gas)?;
         }
         Ok(Vec::new())
     }
diff --git a/src/opcodes.rs b/src/opcodes.rs
index 93aa7a1..7c9ce94 100644
--- a/src/opcodes.rs
+++ b/src/opcodes.rs
@@ -188,7 +188,7 @@ impl Stack {
         Ok(())
     }
     pub fn sdiv(&mut self) -> Result<(), String> {
-        Err(format!("unimplemented"))
+        Err("unimplemented".to_string())
     }
     pub fn modulus(&mut self) -> Result<(), String> {
         let b0 = BigUint::from_bytes_be(&self.pop()?[..]);
@@ -197,7 +197,7 @@ impl Stack {
         Ok(())
     }
     pub fn smod(&mut self) -> Result<(), String> {
-        Err(format!("unimplemented"))
+        Err("unimplemented".to_string())
     }
     pub fn add_mod(&mut self) -> Result<(), String> {
         let b0 = BigUint::from_bytes_be(&self.pop()?[..]);
@@ -334,13 +334,20 @@ impl Stack {
         self.storage.insert(key, value.to_vec());
         Ok(())
     }
-    pub fn jump(&mut self) -> Result<(), String> {
+    pub fn jump(&mut self, code: &[u8]) -> Result<(), String> {
         // TODO that jump destination is valid
-        self.pc = u256::u256_to_u64(self.pop()?) as usize;
+        let new_pc = u256::u256_to_u64(self.pop()?) as usize;
+        if !valid_dest(code, new_pc) {
+            return Err(format!("not valid dest: {:02x}", new_pc));
+        }
+        self.pc = new_pc;
         Ok(())
     }
-    pub fn jump_i(&mut self) -> Result<(), String> {
+    pub fn jump_i(&mut self, code: &[u8]) -> Result<(), String> {
         let new_pc = u256::u256_to_u64(self.pop()?) as usize;
+        if !valid_dest(code, new_pc) {
+            return Err(format!("not valid dest: {:02x}", new_pc));
+        }
         if !self.stack.is_empty() {
             let cond = u256::u256_to_u64(self.pop()?) as usize;
             if cond != 0 {
@@ -357,6 +364,13 @@ impl Stack {
     }
 }
 
+fn valid_dest(code: &[u8], pos: usize) -> bool {
+    if code[pos] == 0x5b {
+        return true;
+    }
+    false
+}
+
 fn upper_multiple_of_32(n: usize) -> usize {
     ((n - 1) | 31) + 1
 }
diff --git a/tests/execute.rs b/tests/execute.rs
index 90c2516..f690dc4 100644
--- a/tests/execute.rs
+++ b/tests/execute.rs
@@ -169,6 +169,29 @@ fn execute_opcodes_7() {
     assert_eq!(out, hex::decode("6005600401").unwrap());
 }
 
+#[test]
+fn execute_exceptions() {
+    let mut s = Stack::new();
+    let calldata = hex::decode("").unwrap();
+
+    let code = hex::decode("5f").unwrap();
+    let out = s.execute(&code, &calldata, false);
+    assert_eq!(out, Err(format!("invalid opcode 5f")));
+
+    let code = hex::decode("56").unwrap();
+    let out = s.execute(&code, &calldata, false);
+    assert_eq!(out, Err(format!("pop err")));
+
+    let code = hex::decode("600056").unwrap();
+    let out = s.execute(&code, &calldata, false);
+    assert_eq!(out, Err(format!("not valid dest: 00")));
+
+    s.gas = 1;
+    let code = hex::decode("6000").unwrap();
+    let out = s.execute(&code, &calldata, false);
+    assert_eq!(out, Err(format!("out of gas")));
+}
+
 #[test]
 fn execute_opcodes_8() {
     let code = hex::decode("611000805151").unwrap();
@@ -190,7 +213,7 @@ fn execute_opcodes_9() {
     let calldata = hex::decode("").unwrap();
 
     let mut s = Stack::new();
-    s.execute(&code, &calldata, true).unwrap();
+    s.execute(&code, &calldata, false).unwrap();
 
     assert_eq!(s.gas, 9999974988);
     // assert_eq!(s.gas, 9999977788); // TODO WIP geth reported gas