If I write the formula "=E3+E4" in a pane that is formated as a number and E3 and E4 also are formated as number, how come that I get a #VALUE!-error when the formula is being calculated?

I get the same error if I write the formula like this: =SUM(E3+E4)

Excel want to treat E3 and E4 as strings not as numbers.

Can you please help me on this?