diff --git a/src/transaction.rs b/src/transaction.rs
index 3c2039ab0..16395c8eb 100644
--- a/src/transaction.rs
+++ b/src/transaction.rs
@@ -129,8 +129,12 @@ impl Transaction {
     pub fn signed_key(&self, instruction_index: usize, accounts_index: usize) -> Option<&Pubkey> {
         match self.key_index(instruction_index, accounts_index) {
             None => None,
-            Some(0) => self.account_keys.get(0),
-            Some(_) => None,
+            Some(signature_index) => {
+                if signature_index >= self.signatures.len() {
+                    return None;
+                }
+                self.account_keys.get(signature_index)
+            }
         }
     }
     pub fn program_id(&self, instruction_index: usize) -> &Pubkey {
@@ -306,22 +310,19 @@ mod tests {
         assert_eq!(
             serialize(&tx).unwrap(),
             vec![
-                1, 0, 0, 0, 0, 0, 0, 0, 199, 111, 28, 46, 13, 235, 122, 203, 210, 118, 158, 0, 59,
-                175, 148, 175, 225, 134, 90, 155, 188, 232, 218, 54, 199, 116, 67, 99, 65, 84, 164,
-                94, 224, 60, 187, 249, 92, 210, 242, 99, 58, 121, 202, 5, 140, 152, 14, 166, 13,
-                134, 103, 127, 216, 96, 217, 139, 5, 252, 40, 146, 48, 112, 59, 7, 1, 0, 0, 0, 0,
+                1, 0, 0, 0, 0, 0, 0, 0, 234, 139, 34, 5, 120, 28, 107, 203, 69, 25, 236, 200, 164,
+                1, 12, 47, 147, 53, 41, 143, 23, 116, 230, 203, 59, 228, 153, 14, 22, 241, 103,
+                226, 186, 169, 181, 65, 49, 215, 44, 2, 61, 214, 113, 216, 184, 206, 147, 104, 140,
+                225, 138, 21, 172, 135, 211, 80, 103, 80, 216, 106, 249, 86, 194, 1, 3, 0, 0, 0, 0,
                 0, 0, 0, 32, 253, 186, 201, 177, 11, 117, 135, 187, 167, 181, 188, 22, 59, 206,
-                105, 231, 150, 215, 30, 78, 212, 76, 16, 252, 180, 72, 134, 137, 247, 161, 68, 3,
-                0, 0, 0, 0, 0, 0, 0, 32, 253, 186, 201, 177, 11, 117, 135, 187, 167, 181, 188, 22,
-                59, 206, 105, 231, 150, 215, 30, 78, 212, 76, 16, 252, 180, 72, 134, 137, 247, 161,
-                68, 32, 253, 186, 201, 177, 11, 117, 135, 187, 167, 181, 188, 22, 59, 206, 105,
-                231, 150, 215, 30, 78, 212, 76, 16, 252, 180, 72, 134, 137, 247, 161, 68, 1, 1, 1,
-                4, 5, 6, 7, 8, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 8, 7, 6, 5, 4, 1, 1,
-                1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-                0, 0, 0, 0, 0, 99, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 2, 2, 2, 4, 5, 6,
-                7, 8, 9, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 9, 8, 7, 6, 5, 4, 2, 2, 2, 1, 0,
-                0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 0, 0, 0, 0, 0, 0, 1, 2, 3, 0, 0, 0, 0, 0, 0, 0, 1, 2,
-                3
+                105, 231, 150, 215, 30, 78, 212, 76, 16, 252, 180, 72, 134, 137, 247, 161, 68, 32,
+                253, 186, 201, 177, 11, 117, 135, 187, 167, 181, 188, 22, 59, 206, 105, 231, 150,
+                215, 30, 78, 212, 76, 16, 252, 180, 72, 134, 137, 247, 161, 68, 1, 1, 1, 4, 5, 6,
+                7, 8, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 8, 7, 6, 5, 4, 1, 1, 1, 0, 0,
+                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+                0, 0, 99, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 2, 2, 2, 4, 5, 6, 7, 8, 9,
+                1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 9, 8, 7, 6, 5, 4, 2, 2, 2, 1, 0, 0, 0, 0,
+                0, 0, 0, 0, 3, 0, 0, 0, 0, 0, 0, 0, 0, 1, 2, 3, 0, 0, 0, 0, 0, 0, 0, 1, 2, 3
             ],
         );
     }