diff --git a/src/script.js b/src/script.js
index 7c4aad1..cc20182 100644
--- a/src/script.js
+++ b/src/script.js
@@ -199,23 +199,32 @@ function toBuffer (input) {
   return bsv.Script.fromASM(text.replace(/\s+/g, ' ')).toBuffer()
 }
 
-// Whether a push anywhere in buf, including after OP_RETURN, runs past its end.
-function truncated (buf) {
+// What in buf, including the data after OP_RETURN, ASM cannot hold: a push
+// that runs past the end, a push not written with the shortest opcode for its
+// length (ASM records only the data), or an opcode with no name.
+function asmObstacles (buf) {
+  const found = { truncated: false, nonMinimal: false, unnamed: false }
   let i = 0
   while (i < buf.length) {
     const code = buf[i++]
-    if (code === 0 || code > OP.OP_PUSHDATA4) continue
+    if (code > OP.OP_PUSHDATA4) {
+      if (!NAMES[code]) found.unnamed = true
+      continue
+    }
+    if (code === 0) continue
     let len = code
     if (code >= OP.OP_PUSHDATA1) {
       const w = code === OP.OP_PUSHDATA1 ? 1 : code === OP.OP_PUSHDATA2 ? 2 : 4
-      if (i + w > buf.length) return true
+      if (i + w > buf.length) { found.truncated = true; break }
       len = w === 1 ? buf[i] : w === 2 ? buf.readUInt16LE(i) : buf.readUInt32LE(i)
       i += w
+      const shortest = len === 0 ? 0 : len < OP.OP_PUSHDATA1 ? len : len < 0x100 ? OP.OP_PUSHDATA1 : len < 0x10000 ? OP.OP_PUSHDATA2 : OP.OP_PUSHDATA4
+      if (shortest !== code) found.nonMinimal = true
     }
-    if (i + len > buf.length) return true
+    if (i + len > buf.length) { found.truncated = true; break }
     i += len
   }
-  return false
+  return found
 }
 
 // ASM that reads back as exactly `buf`, or an error. ASM cannot say how data
@@ -227,12 +236,12 @@ function exactAsm (buf) {
   let back = null
   try { back = toBuffer(asm) } catch (e) {}
   if (!back || !back.equals(buf)) {
-    const ops = parse(buf, { deadBlocks: false })
-    const why = truncated(buf)
+    const found = asmObstacles(buf)
+    const why = found.truncated
       ? 'it contains a truncated push'
-      : ops.some(o => isPush(o) && o.code !== TAIL && opSize(pushOp(pushValue(o))) !== opSize(o))
+      : found.nonMinimal
         ? 'a push is not minimally encoded, and ASM cannot say how data was pushed'
-        : ops.some(o => o.code >= 0 && !isPush(o) && !NAMES[o.code])
+        : found.unnamed
           ? 'it uses an opcode that has no name, which @smartledger/bsv reads back from ASM only from 9.11.1'
           : !/\bOP_/.test(asm)
               ? 'it is only data pushes, and ASM with no opcodes reads back as hex'
diff --git a/test/unit.test.js b/test/unit.test.js
index 2a8585e..ea7e8ed 100644
--- a/test/unit.test.js
+++ b/test/unit.test.js
@@ -204,6 +204,7 @@ test('ASM output reads back byte for byte, or is refused', () => {
   // An empty push in OP_RETURN data, as buildDataOut('') writes it (read back from bsv 9.11.2).
   assert.strictEqual(exactAsm(Buffer.from('516a00020aff', 'hex')), 'OP_1 OP_RETURN 0 0aff')
   assert.throws(() => exactAsm(Buffer.from('516a05aabb', 'hex')), /truncated push/)
+  assert.throws(() => exactAsm(Buffer.from('516a4c00', 'hex')), /not minimally encoded/)
   assert.throws(() => exactAsm(Buffer.from('4c50' + 'ab'.repeat(80), 'hex')), /only data pushes/)
 
   const bin = path.join(__dirname, '..', 'bin', 'scriptmin.js')
@@ -215,7 +216,7 @@ test('ASM output reads back byte for byte, or is refused', () => {
     const written = require('fs').readFileSync(path.join(dir, 'out.asm'), 'utf8')
     assert.ok(written.trim().endsWith('OP_RETURN ' + 'ab'.repeat(40)), written)
     require('fs').writeFileSync(path.join(dir, 'odd.hex'), '76756a4c03aabbcc')
-    assert.throws(() => execFileSync('node', [bin, '--asm', '--tests', '0', '-o', path.join(dir, 'odd.asm'), path.join(dir, 'odd.hex')], { stdio: 'pipe' }), e => e.status === 2 && /write it as hex/.test(e.stderr))
+    assert.throws(() => execFileSync('node', [bin, '--asm', '--tests', '0', '-o', path.join(dir, 'odd.asm'), path.join(dir, 'odd.hex')], { stdio: 'pipe' }), e => e.status === 2 && /not minimally encoded/.test(e.stderr))
   } finally {
     require('fs').rmSync(dir, { recursive: true, force: true })
   }