diff --git a/txscript/script.go b/txscript/script.go
index 112c24ac..58b8e69f 100644
--- a/txscript/script.go
+++ b/txscript/script.go
@@ -1,4 +1,5 @@
 // Copyright (c) 2013-2017 The btcsuite developers
+// Copyright (c) 2015-2019 The Decred developers
 // Use of this source code is governed by an ISC
 // license that can be found in the LICENSE file.
 
@@ -45,11 +46,12 @@ const (
 
 // isSmallInt returns whether or not the opcode is considered a small integer,
 // which is an OP_0, or OP_1 through OP_16.
-func isSmallInt(op *opcode) bool {
-	if op.value == OP_0 || (op.value >= OP_1 && op.value <= OP_16) {
-		return true
-	}
-	return false
+//
+// NOTE: This function is only valid for version 0 opcodes.  Since the function
+// does not accept a script version, the results are undefined for other script
+// versions.
+func isSmallInt(op byte) bool {
+	return op == OP_0 || (op >= OP_1 && op <= OP_16)
 }
 
 // isScriptHash returns true if the script passed is a pay-to-script-hash
@@ -136,7 +138,7 @@ func IsWitnessProgram(script []byte) bool {
 // bytes.
 func isWitnessProgram(pops []parsedOpcode) bool {
 	return len(pops) == 2 &&
-		isSmallInt(pops[0].opcode) &&
+		isSmallInt(pops[0].opcode.value) &&
 		canonicalPush(pops[1]) &&
 		(len(pops[1].data) >= 2 && len(pops[1].data) <= 40)
 }
diff --git a/txscript/standard.go b/txscript/standard.go
index 2cad218e..a447303d 100644
--- a/txscript/standard.go
+++ b/txscript/standard.go
@@ -115,10 +115,10 @@ func isMultiSig(pops []parsedOpcode) bool {
 	if l < 4 {
 		return false
 	}
-	if !isSmallInt(pops[0].opcode) {
+	if !isSmallInt(pops[0].opcode.value) {
 		return false
 	}
-	if !isSmallInt(pops[l-2].opcode) {
+	if !isSmallInt(pops[l-2].opcode.value) {
 		return false
 	}
 	if pops[l-1].opcode.value != OP_CHECKMULTISIG {
@@ -153,7 +153,7 @@ func isNullData(pops []parsedOpcode) bool {
 
 	return l == 2 &&
 		pops[0].opcode.value == OP_RETURN &&
-		(isSmallInt(pops[1].opcode) || pops[1].opcode.value <=
+		(isSmallInt(pops[1].opcode.value) || pops[1].opcode.value <=
 			OP_PUSHDATA4) &&
 		len(pops[1].data) <= MaxDataCarrierSize
 }
@@ -705,7 +705,7 @@ func ExtractAtomicSwapDataPushes(version uint16, pkScript []byte) (*AtomicSwapDa
 			return nil, nil
 		}
 		pushes.SecretSize = int64(locktime)
-	} else if op := pops[2].opcode; isSmallInt(op) {
+	} else if op := pops[2].opcode; isSmallInt(op.value) {
 		pushes.SecretSize = int64(asSmallInt(op))
 	} else {
 		return nil, nil
@@ -716,7 +716,7 @@ func ExtractAtomicSwapDataPushes(version uint16, pkScript []byte) (*AtomicSwapDa
 			return nil, nil
 		}
 		pushes.LockTime = int64(locktime)
-	} else if op := pops[11].opcode; isSmallInt(op) {
+	} else if op := pops[11].opcode; isSmallInt(op.value) {
 		pushes.LockTime = int64(asSmallInt(op))
 	} else {
 		return nil, nil