Grammatical function of “voller”?

Question

In a test I found the phrase >Sie ist voller Licht which meant (it was said) *she is full of light*. I do not undestand *voller* in this context. Pons says it is an *Inv. adj.*, which, in fact, adds to my confusion. Langenscheidt writes *voll(er)* with no indication about the use or not of the *er* ending.

Answer 1

I believe this is due to the fact the it can be challenging to pronounce the two 'L' sounds in quick succession, similar to the way English speakers say "a dog" but "an anaconda": Because saying "a anaconda" is not as smooth to pronounce due to the two "A" sounds in quick succession.