Squid Proxy: 400 Bad Request when "%25" (Percent Sign) in URL

I have a squid proxy that works well except for this issue:

If a URL has a %25 inside of it (the percent sign), we get a 400 Bad Request and Bad Request is displayed to the web browser.

Example URL:
http://www.amazon.com/25%25-Percent-Off-Stickers-Adhesive/dp/B00J0IBJ0S/

Log:

12/Jan/2016:18:40:28 -0600 429 MY.IP.IS.HERE TCP_MISS/400 310 GET http://www.amazon.com/25%25-Percent-Off-Stickers-Adhesive/dp/B00J0IBJ0S/ – ROUNDROBIN_PARENT/three text/html

I’m not sure if this is a bug or a configuration error. I have a round robin setup as shown above. Here is the output of squid3 -v:

Squid Cache: Version 3.1.19 configure options:
‘–build=x86_64-linux-gnu’ ‘–prefix=/usr’
‘–includedir=${prefix}/include’ ‘–mandir=${prefix}/share/man’
‘–infodir=${prefix}/share/info’ ‘–sysconfdir=/etc’
‘–localstatedir=/var’ ‘–libexecdir=${prefix}/lib/squid3’
‘–srcdir=.’ ‘–disable-maintainer-mode’
‘–disable-dependency-tracking’ ‘–disable-silent-rules’
‘–datadir=/usr/share/squid3’ ‘–sysconfdir=/etc/squid3’
‘–mandir=/usr/share/man’ ‘–with-cppunit-basedir=/usr’
‘–enable-inline’ ‘–enable-async-io=8’
‘–enable-storeio=ufs,aufs,diskd’ ‘–enable-removal-policies=lru,heap’
‘–enable-delay-pools’ ‘–enable-cache-digests’ ‘–enable-underscores’
‘–enable-icap-client’ ‘–enable-follow-x-forwarded-for’
‘–enable-auth=basic,digest,ntlm,negotiate’
‘–enable-basic-auth-helpers=LDAP,MSNT,NCSA,PAM,SASL,SMB,YP,DB,POP3,getpwnam,squid_radius_auth,multi-domain-NTLM’
‘–enable-ntlm-auth-helpers=smb_lm,’
‘–enable-digest-auth-helpers=ldap,password’
‘–enable-negotiate-auth-helpers=squid_kerb_auth’
‘–enable-external-acl-helpers=ip_user,ldap_group,session,unix_group,wbinfo_group’
‘–enable-arp-acl’ ‘–enable-esi’ ‘–enable-zph-qos’ ‘–enable-wccpv2’
‘–disable-translation’ ‘–with-logdir=/var/log/squid3’
‘–with-pidfile=/var/run/squid3.pid’ ‘–with-filedescriptors=65536’
‘–with-large-files’ ‘–with-default-user=proxy’
‘–enable-linux-netfilter’ ‘build_alias=x86_64-linux-gnu’ ‘CFLAGS=-g
-O2 -fPIE -fstack-protector –param=ssp-buffer-size=4 -Wformat -Wformat-security -Werror=format-security’ ‘LDFLAGS=-Wl,-Bsymbolic-functions -fPIE -pie -Wl,-z,relro -Wl,-z,now’
‘CPPFLAGS=-D_FORTIFY_SOURCE=2’ ‘CXXFLAGS=-g -O2 -fPIE
-fstack-protector –param=ssp-buffer-size=4 -Wformat -Wformat-security -Werror=format-security’ –with-squid=/build/squid3-FzlLQ3/squid3-3.1.19

uname -a:

Linux MyHostName 3.13.0-44-generic #73~precise1-Ubuntu SMP Wed Dec 17 00:39:15 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

(It’s an Ubuntu 12.04.5 LTS server)

The hack with this URL would simply be to have something strip out the %25 but that wouldn’t work on all destination websites.

The URL works fine when not going through the proxy.

Thanks for any ideas, I’m willing to provide more config info.