ISE in Content Assist Parser

Given the grammar

Model:
	'model' name=Identifier '{'(
		(b=B)? &
		(cl=CList)?
	)'}';
B: {B}'b' '{'  '}';
CList:
	-> c+=C+;
C:
	{C}
	'c' sr+=SomeReference ('+' sr+=SomeReference)* '{'
	 xc1b=SomeEnum?
	'}';
SomeReference: target=Identifier;
enum SomeEnum: bla='Bla' | blubb='Blubb';
Identifier: ID;

and the sample model

model MyModel {
	c A+B+C {}
	c D{Blubb}
	c <|>
	b{}
}

with <|> beeing the cursor content assist gives

Caused by: java.lang.reflect.InvocationTargetException
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.eclipse.xtext.ide.editor.contentassist.antlr.BaseContentAssistParser.getFollowElements(BaseContentAssistParser.java:171)
	at org.eclipse.xtext.ide.editor.contentassist.antlr.BaseContentAssistParser.getFollowElements(BaseContentAssistParser.java:241)
	... 12 more
Caused by: java.lang.IllegalStateException: expected element: 'org.eclipse.xtext.impl.RuleCallImpl@1edcfb88 (cardinality: null, predicated: false, firstSetPredicated: false) (explicitlyCalled: false)', but was: 'org.eclipse.xtext.impl.AssignmentImpl@680410ec (cardinality: +, predicated: false, firstSetPredicated: true) (feature: c, operator: +=)'
	at org.eclipse.xtext.ide.editor.contentassist.antlr.internal.BaseInternalContentAssistParser.after(BaseInternalContentAssistParser.java:153)
	at org.xtext.example.mydsl1.ide.contentassist.antlr.internal.InternalMyDslParser.rule__Model__ClAssignment_3_1(InternalMyDslParser.java:2659)
	at org.xtext.example.mydsl1.ide.contentassist.antlr.internal.InternalMyDslParser.rule__Model__UnorderedGroup_3__Impl(InternalMyDslParser.java:2411)
	at org.xtext.example.mydsl1.ide.contentassist.antlr.internal.InternalMyDslParser.rule__Model__UnorderedGroup_3__0(InternalMyDslParser.java:2467)
	at org.xtext.example.mydsl1.ide.contentassist.antlr.internal.InternalMyDslParser.rule__Model__UnorderedGroup_3(InternalMyDslParser.java:2281)
	at org.xtext.example.mydsl1.ide.contentassist.antlr.internal.InternalMyDslParser.rule__Model__Group__3__Impl(InternalMyDslParser.java:1069)
	at org.xtext.example.mydsl1.ide.contentassist.antlr.internal.InternalMyDslParser.rule__Model__Group__3(InternalMyDslParser.java:1022)
	at org.xtext.example.mydsl1.ide.contentassist.antlr.internal.InternalMyDslParser.rule__Model__Group__2(InternalMyDslParser.java:948)
	at org.xtext.example.mydsl1.ide.contentassist.antlr.internal.InternalMyDslParser.rule__Model__Group__1(InternalMyDslParser.java:859)
	at org.xtext.example.mydsl1.ide.contentassist.antlr.internal.InternalMyDslParser.rule__Model__Group__0(InternalMyDslParser.java:780)
	at org.xtext.example.mydsl1.ide.contentassist.antlr.internal.InternalMyDslParser.ruleModel(InternalMyDslParser.java:134)