textcomp Error: symbol \textuparrow not provided by

[Sam Liddicott]

Having just added a symbol up-arrow, every time I build my PDF, Lyx 
gives this error:

textcomp Error: symbol \textuparrow not provided by
("by" is the last word of the error)

althought the PDF displays the arrow just fine.

The arrow is in a LyxCode environment, but further testing shows that it 
just hates doing an up-arrow within \ttfamily

as it works anyway, any tips to stop it complaining?

Sam