Zorba error [zerr:ZXQP0002]: "theBodyExpr": assertion failed.

Compiling the following query:
module namespace guestbook = "http://www.28msec.com/templates/guestbook/guestbook";

import module namespace functx = "http://www.functx.com/";

import module namespace db = "http://www.zorba-xquery.com/modules/store/static/collections/dml";
import module namespace idx = "http://www.zorba-xquery.com/modules/store/static/indexes/dml";

declare namespace an = "http://www.zorba-xquery.com/annotations";

declare collection guestbook:entries as node()*;
declare variable $guestbook:entries as xs:QName := xs:QName("guestbook:entries");

(: Access a document range :)
declare %private %an:automatic %an:unique %an:value-range index guestbook:by-date
  on nodes db:collection(xs:QName('guestbook:entries'))
  by guestbook:convert-dateTime(@datetime) as xs:dateTime;
declare %private variable $guestbook:by-date as xs:QName := xs:QName('guestbook:by-date');

declare function guestbook:convert-dateTime($dateTime as xs:string)
as xs:dateTime {
  current-dateTime()
};

Results in

0 libzorba_simplestore.2.5.0.dylib 0x000000010932780a _ZN5zorba16assertion_failedEPKcS1_iS1_ + 74
1 libzorba_simplestore.2.5.0.dylib 0x0000000109488cbf _ZN5zorba13user_function8optimizeEPNS_10CompilerCBE + 1259
2 libzorba_simplestore.2.5.0.dylib 0x000000010921ab66 _ZN5zorba9MarkExprs5applyERNS_15RewriterContextEPNS_4exprERb + 150
3 libzorba_simplestore.2.5.0.dylib 0x000000010920ab69 _ZN5zorba15RuleMajorDriver7rewriteERNS_15RewriterContextE + 125
4 libzorba_simplestore.2.5.0.dylib 0x000000010920bdc2 _ZN5zorba16DefaultOptimizer7rewriteERNS_15RewriterContextE + 304
5 libzorba_simplestore.2.5.0.dylib 0x000000010914d938 _ZN5zorba14TranslatorImpl9end_visitERKNS_12IndexKeyListEPv + 5482
6 libzorba_simplestore.2.5.0.dylib 0x00000001090f706c _ZNK5zorba13AST_IndexDecl6acceptERNS_17parsenode_visitorE + 78
7 libzorba_simplestore.2.5.0.dylib 0x00000001090fd4a2 _ZNK5zorba12VFO_DeclList6acceptERNS_17parsenode_visitorE + 64
8 libzorba_simplestore.2.5.0.dylib 0x00000001090f5b12 _ZNK5zorba6Prolog6acceptERNS_17parsenode_visitorE + 78
9 libzorba_simplestore.2.5.0.dylib 0x00000001090f5a9a _ZNK5zorba13LibraryModule6acceptERNS_17parsenode_visitorE + 96
10 libzorba_simplestore.2.5.0.dylib 0x000000010912afa9 _ZN5zorbaL13translate_auxEPNS_14TranslatorImplERKNS_9parsenodeEPNS_14static_contextEmPNS_11ModulesInfoERKSt3mapINS_7rstringINS_15rstring_classes3repINS_10atomic_intESt11char_traitsIcESaIcEEEEESI_St4lessISI_ESaISt4pairIKSI_SI_EEEbNS_19StaticContextConsts16xquery_version_tE + 105
11 libzorba_simplestore.2.5.0.dylib 0x0000000109142381 _ZN5zorba14TranslatorImpl9end_visitERKNS_12ModuleImportEPv + 7243
12 libzorba_simplestore.2.5.0.dylib 0x00000001090fd432 _ZNK5zorba13SIND_DeclList6acceptERNS_17parsenode_visitorE + 206
13 libzorba_simplestore.2.5.0.dylib 0x00000001090f5b00 _ZNK5zorba6Prolog6acceptERNS_17parsenode_visitorE + 60
14 libzorba_simplestore.2.5.0.dylib 0x00000001090f59fe _ZNK5zorba10MainModule6acceptERNS_17parsenode_visitorE + 78
15 libzorba_simplestore.2.5.0.dylib 0x000000010912afa9 _ZN5zorbaL13translate_auxEPNS_14TranslatorImplERKNS_9parsenodeEPNS_14static_contextEmPNS_11ModulesInfoERKSt3mapINS_7rstringINS_15rstring_classes3repINS_10atomic_intESt11char_traitsIcESaIcEEEEESI_St4lessISI_ESaISt4pairIKSI_SI_EEEbNS_19StaticContextConsts16xquery_version_tE + 105
16 libzorba_simplestore.2.5.0.dylib 0x000000010912ad3e _ZN5zorba9translateERKNS_9parsenodeEPNS_10CompilerCBE + 446
17 libzorba_simplestore.2.5.0.dylib 0x00000001092344b6 _ZN5zorba14XQueryCompiler9normalizeENS_8rchandleINS_9parsenodeEEE + 34
18 libzorba_simplestore.2.5.0.dylib 0x00000001092345cc _ZN5zorba14XQueryCompiler7compileERKNS_8rchandleINS_9parsenodeEEEbRNS1_INS_4exprEEERmRNS_5audit12ScopedRecordE + 130
19 libzorba_simplestore.2.5.0.dylib 0x00000001092355a8 _ZN5zorba14XQueryCompiler7compileERSiRKNS_7rstringINS_15rstring_classes3repINS_10atomic_intESt11char_traitsIcESaIcEEEEERm + 534
20 libzorba_simplestore.2.5.0.dylib 0x0000000109065551 _ZN5zorba10XQueryImpl9doCompileERSiRK19Zorba_CompilerHintsbRm + 861
21 libzorba_simplestore.2.5.0.dylib 0x0000000109065cce _ZN5zorba10XQueryImpl7compileERSiRKNS_8SmartPtrINS_13StaticContextEEERK19Zorba_CompilerHints + 518
22 zorba 0x00000001090437b9 _Z17compileAndExecutePN5zorba5ZorbaERK18ZorbaCMDPropertiesRNS_8SmartPtrINS_13StaticContextEEERKSsRSiRSoR10TimingInfo + 1705
23 zorba 0x000000010904592c main + 4108
24 zorba 0x0000000109041e94 start + 52