diff --git a/lib/Crypt/Misc.pm b/lib/Crypt/Misc.pm
index 0782e73..79951bd 100644
--- a/lib/Crypt/Misc.pm
+++ b/lib/Crypt/Misc.pm
@@ -64,7 +64,7 @@
 
   my $default = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuv";
   if (defined $alphabet) {
-    return undef if $alphabet !~ /^[a-zA-Z0-9]{58}$/;
+    return undef if $alphabet !~ /^[a-zA-Z0-9]{58}$/ || $base58 !~ /^[$alphabet]+$/;
     eval "\$base58 =~ tr/$alphabet/$default/"; # HACK: https://stackoverflow.com/questions/11415045/using-a-char-variable-in-tr
     return undef if $@;
   }
diff --git a/t/crypt-misc.t b/t/crypt-misc.t
index 9cd323d..d8892f3 100644
--- a/t/crypt-misc.t
+++ b/t/crypt-misc.t
@@ -1,6 +1,6 @@
 use strict;
 use warnings;
-use Test::More tests => 679;
+use Test::More tests => 680;
 
 use Crypt::Misc qw( encode_b64   decode_b64
                     encode_b64u  decode_b64u
@@ -143,3 +143,5 @@
   is(unpack("H*", decode_b32z(encode_b32z($b))), $h);
   is(unpack("H*", decode_b32c(encode_b32c($b))), $h);
 }
+
+is(decode_b58b("111OIl0"), undef, "bug: decode_b58b + invalid input");