Iconic painting The Scream by Edvard Munch, along with his The Madonna, which were stolen from the Munch Museum in Oslo last weekend, were not insured for theft.
David Scully, underwriting director for AXA Art, said it was common practice for some of the world's most famous art works to be uninsured for theft.
Scully said many public galleries could not afford to insure their most famous pieces, and theft cover was often not taken out as works of art were irreplaceable.
The paintings were insured for damages from fire of flooding through Oslo Forsikring, the museum's insurer.
Scully said it was more common for art to be covered for fire and flood damage, as money would help fund restoration.
Hiscox fine art underwriter Robert Read said armed robbery was very unusual in cases such as these, but that if it did become common, it would be “a worrying trend”.