diff --git a/src/script/sighashtype.h b/src/script/sighashtype.h
--- a/src/script/sighashtype.h
+++ b/src/script/sighashtype.h
@@ -20,7 +20,7 @@
 };
 
 /**
- * Base signature hash types
+ * Base signature hash types nominally range from 0x00 to 0x1f.
  * Base sig hash types not defined in this enum may be used, but they will be
  * represented as UNSUPPORTED.  See transaction
  * c99c49da4c38af669dea436d3e73780dfdb6c1ecf9958baa52960e8baee30e73 for an
@@ -67,11 +67,17 @@
 
     uint32_t getForkValue() const { return sigHash >> 8; }
 
+    // Checks whether baseType has supported value AND bit 0x20 is unset, i.e,
+    // make sure that (sigHash&0x3f) is either ALL, SINGLE, or NONE.
     bool isDefined() const {
-        auto baseType =
+        // While BaseSigHashType normally ranges from 0x00 to 0x1f, here
+        // lowBits ranges from 0x00 to 0x3f due to the narrowing conversion
+        // (BaseSigHashType is uint8_t, implicitly clipping forkID) and
+        // omission of bit 0x20 from the mask.
+        auto lowBits =
             BaseSigHashType(sigHash & ~(SIGHASH_FORKID | SIGHASH_ANYONECANPAY));
-        return baseType >= BaseSigHashType::ALL &&
-               baseType <= BaseSigHashType::SINGLE;
+        return lowBits >= BaseSigHashType::ALL &&
+               lowBits <= BaseSigHashType::SINGLE;
     }
 
     bool hasForkId() const { return (sigHash & SIGHASH_FORKID) != 0; }