diff --git a/lib/encoding/base64/base64_test.ww b/lib/encoding/base64/base64_test.ww
index cd0d9bce..61fb0ebb 100644
--- a/lib/encoding/base64/base64_test.ww
+++ b/lib/encoding/base64/base64_test.ww
@@ -117,15 +117,32 @@ fn inval_check(enc: *base64.encoding, encoded: str) void = {
 
 // ---- decodestr error cases ---- ref/hare/encoding/base64/base64.ha:525
 //
-// Wrong length, bad char, embedded / excess padding all → invalid.
+// Table-driven (parallel-array idiom; tuple rows blocked by #111). Each
+// row exercises one malformed class the hand-written validation in
+// decodestr must reject. Cross-alphabet chars are covered separately by
+// urlsafe_distinct(); Hare's all-padding-but-not-multiple-of-4 rows
+// ("=", "===", "=====", base64.ha:527-531) collapse onto the length
+// guard here, so this table pins the multiple-of-4 survivors of every
+// class instead.
 
 @test fn decode_invalid() void = {
-	inval_check(&base64.std_encoding, "Zg");        // not a multiple of 4
-	inval_check(&base64.std_encoding, "Z@==");      // '@' not in alphabet
-	inval_check(&base64.std_encoding, "====");      // all padding
-	inval_check(&base64.std_encoding, "Zg==Zg==");  // data after padding
-	inval_check(&base64.std_encoding, "Zm8=Zm8=");  // data after padding
-	inval_check(&base64.std_encoding, "@Zg=");      // bad leading char
+	let bad: [10]str = [
+		"Z",         // len % 4 == 1 (lone trailing char)
+		"Zg",        // len % 4 == 2
+		"Zm9",       // len % 4 == 3
+		"Z@==",      // non-alphabet char in data region
+		"@Zg=",      // bad leading char
+		"====",      // all padding (no data)
+		"A===",      // excess '=' (3 pads in the final quad)
+		"Z=g=",      // '=' not in the trailing position
+		"Zg==Zg==",  // data after padding (embedded '=')
+		"Zm8=Zm8=",  // data after padding (embedded '=')
+	];
+	let i: i32 = 0;
+	for (i < len(bad)) {
+		inval_check(&base64.std_encoding, bad[i]);
+		i += 1;
+	};
 };
 
 // ---- size calc ---- ref/hare/encoding/base64/base64.ha:601